[IMP] Taxes: improve tax configuration doc

content/applications/finance/accounting/taxes.rst

@@ -72,23 +72,42 @@ The **tax name** is displayed for backend users in the :guilabel:`Taxes` field i
 Tax computation
 ~~~~~~~~~~~~~~~
 
-- **Group of Taxes**
+The :guilabel:`Tax Computation` field determines how the tax amount is computed from the sales
+price. The following options are available:
+
+- :ref:`Group of Taxes <taxes/computation/group-of-taxes>`: a combination of several other taxes
+- :ref:`Fixed <taxes/computation/fixed>`: a fixed amount
+- :ref:`Percentage of Price <taxes/computation/percentage-of-price>`: a percentage of the
+  tax-excluded sales price
+- :ref:`Percentage of Price Tax Included <taxes/computation/percentage-of-price-tax-included>`: a
+  percentage of the tax-included total
+- :ref:`Custom Formula <taxes/computation/python-code>`: a custom, user-defined formula
+
+.. _taxes/computation/group-of-taxes:
+
+Group of taxes
+**************
 
   The tax is a combination of multiple sub-taxes. You can add as many taxes as you want, in the
   order you want them to be applied.
 
-  .. important::
-     Make sure that the tax sequence is correct, as the order in which they are may impact the
-     taxes' amounts computation, especially if one of the taxes :ref:`affects the base of the
-     subsequent ones <taxes/base-subsequent>`.
+.. important::
+   Make sure the tax sequence is correct, as the display order determines the application order and
+   may affect tax computation, particularly if a tax :ref:`affects the base of subsequent taxes
+   <taxes/base-subsequent>`.
+
+.. _taxes/computation/fixed:
 
-- **Fixed**
+Fixed
+*****
 
-  The tax has a fixed amount in the default currency. The amount remains the same, regardless of the
-  sales price.
+The tax has a fixed amount in the default currency. The amount remains the same per unit,
+regardless of the sales price.
+
+The computation is :math:`\text{tax amount} = \text{fixed tax amount} \times \text{quantity}`.
 
 .. example::
-   A product has a sales price of $1000, and we apply a $10 *fixed* tax. We then have:
+   A product has a sales price of $1000, and we apply a $10 :guilabel:`Fixed` tax. We then have:
 
    +-------------+-------------+----------+----------+
    | Product     | Price       | Tax      | Total    |
@@ -97,47 +116,150 @@ Tax computation
    | 1,000       | 1,000       | 10       | 1,010.00 |
    +-------------+-------------+----------+----------+
 
-- **Percentage of price**
+.. _taxes/computation/percentage-of-price:
 
-  The *sales price* is the taxable basis: the tax amount is computed by multiplying the sales price
-  by the tax percentage.
+Percentage of price
+*******************
 
-.. example::
-   A product has a sales price of $1000, and we apply a *10% of Price* tax. We then have:
+The tax rate is a percentage of the **tax-excluded** subtotal.
 
-   +-------------+-------------+----------+----------+
-   | Product     | Price       | Tax      | Total    |
-   | sales price | without tax |          |          |
-   +=============+=============+==========+==========+
-   | 1,000       | 1,000       | 100      | 1,100.00 |
-   +-------------+-------------+----------+----------+
+The exact tax computation depends on the :ref:`Included in Price <taxes/included-in-price>` field,
+which determines whether the sales price should be treated as tax-excluded or tax-included:
+
+.. tabs::
+   .. tab:: Tax-excluded
+
+      If :guilabel:`Included in Price` is :guilabel:`Tax Excluded`, the computation is
+      :math:`\text{tax amount} = \text{sales price} \times \text{tax rate}`.
+
+      .. example::
+         A product has a sales price of $1000, and we apply a 10% :guilabel:`Percentage of Price`
+         tax that is :guilabel:`Tax Excluded`. We then have:
+
+         +-------------+-------------+----------+----------+
+         | Product     | Price       | Tax      | Total    |
+         | sales price | without tax |          |          |
+         +=============+=============+==========+==========+
+         | 1,000       | 1,000       | 100      | 1,100.00 |
+         +-------------+-------------+----------+----------+
+
+   .. tab:: Tax-included
+
+      If :guilabel:`Included in Price` is :guilabel:`Tax Included`, the computation is
+      :math:`\text{tax amount} = \text{sales price} \times \frac{\text{tax rate}}{1 +
+      \text{tax rate}}`.
+
+      .. example::
+         A product has a sales price of $1000, and we apply a 10% :guilabel:`Percentage of Price`
+         tax that is :guilabel:`Tax Included`. We then have:
+
+         +-------------+-------------+----------+----------+
+         | Product     | Price       | Tax      | Total    |
+         | sales price | without tax |          |          |
+         +=============+=============+==========+==========+
+         | 1,000       | 909.09      | 90.91    | 1,000.00 |
+         +-------------+-------------+----------+----------+
+
+.. _taxes/computation/percentage-of-price-tax-included:
+
+Percentage of price tax included
+********************************
+
+.. important::
+   This tax computation is rarely used and only useful in countries (e.g., Brazil, Bolivia) that
+   quote tax rates as a percentage of the tax-included total.
+   For the more common need to compute tax amounts from a tax-included price, use the
+   :ref:`Percentage of Price <taxes/computation/percentage-of-price>` tax computation with
+   :ref:`Included in Price <taxes/included-in-price>` set to :guilabel:`Tax Included`.
+
+The tax rate is a percentage of the **tax-included** total.
+
+The exact tax computation depends on the :ref:`Included in Price <taxes/included-in-price>` field,
+which determines whether the sales price should be treated as tax-excluded or tax-included:
+
+.. tabs::
+   .. tab:: Tax-excluded
+      If :guilabel:`Included in Price` is set to :guilabel:`Tax Excluded`, the computation is
+      :math:`\text{tax amount} = \text{sales price} \times \frac{\text{tax rate}}{1 -
+      \text{tax rate}}`.
+
+      .. example::
+         A product has a sales price of $1000, and we apply a 10% :guilabel:`Percentage of Price
+         Tax Included` tax that is :guilabel:`Tax Excluded`. We then have:
+
+         +-------------+-------------+----------+----------+
+         | Product     | Price       | Tax      | Total    |
+         | sales price | without tax |          |          |
+         +=============+=============+==========+==========+
+         | 1,000       | 1,000       | 111.11   | 1,111.11 |
+         +-------------+-------------+----------+----------+
+
+         Note that the real tax rate in terms of the tax-excluded price is
+         :math:`\frac{111.11}{1000} = 11.111\%`.
+
+   .. tab:: Tax-included
+
+      If :guilabel:`Included in Price` is set to :guilabel:`Tax Included`, the computation is
+      :math:`\text{tax amount} = \text{sales price} \times \text{tax rate}`.
+
+      .. example::
+         A product has a sales price of $1000, and we apply a 10%
+         :guilabel:`Percentage of Price Tax Included` tax that is :guilabel:`Tax Included`.
+         We then have:
+
+         +-------------+-------------+----------+----------+
+         | Product     | Price       | Tax      | Total    |
+         | sales price | without tax |          |          |
+         +=============+=============+==========+==========+
+         | 1,000       | 900         | 100      | 1,000.00 |
+         +-------------+-------------+----------+----------+
 
-- **Percentage of Price Tax Included**
+         Note that the real tax rate in terms of the tax-excluded price is
+         :math:`\frac{100}{900} = 11.111\%`.
 
-  The **total** is the taxable basis: the tax amount is a percentage of the total.
+.. _taxes/computation/python-code:
+
+Custom formula
+**************
+
+.. important::
+   If a tax can be expressed as a multiple of the quantity of the product to which it applies, it
+   can be defined as a :ref:`Fixed <taxes/computation/fixed>` tax. Doing so is strongly recommended
+   over defining a :guilabel:`Custom Formula` tax.
+
+.. note::
+   To use :guilabel:`Custom Formula` taxes, :ref:`install <general/install>` the :guilabel:`Define
+   Taxes as Python Code` (`account_python_tax`) module.
+
+For a :guilabel:`Custom Formula` tax, the tax amount is computed according to a Python expression
+defined in the :guilabel:`Formula` field. The Python expression may contain the following tokens:
+
+- any of the following variables:
+
+  - `price_unit`: the unit price of the product
+  - `base`: the taxable basis on which the tax is applied - may differ from the `price_unit` if
+    other taxes are applied first
+  - `quantity`: the quantity of the product
+  - `product`: the product record - product fields can also be accessed
+
+- integers and floating-point numbers
+
+- the following permitted tokens: `(`, `)`, `+`, `-`, `*`, `/`, `,`, `<`, `>`, `<=`, `>=`, `and`,
+  `or`, `None`, `min`, and `max`
 
 .. example::
-   A product has a Sales Price of $1000, and we apply a *10% of Price Tax Included* tax. We then
-   have:
+   A product has a sales price of $1000, and we apply a :guilabel:`Custom Formula` tax with a
+   :guilabel:`Formula` of `min(base, 500) * 0.10 + max(base - 500, 0) * 0.20`
+
+   We then have:
 
    +-------------+-------------+----------+----------+
    | Product     | Price       | Tax      | Total    |
    | sales price | without tax |          |          |
    +=============+=============+==========+==========+
-   | 1,000       | 1,000       | 111.11   | 1,111.11 |
+   | 1,000       | 1,000       | 150      | 1,150.00 |
    +-------------+-------------+----------+----------+
 
-- **Python code**
-
-  A tax defined as **Python code** consists of two snippets of Python code that are executed in a
-  local environment containing data such as the unit price, product or partner.
-  :guilabel:`Python Code` defines the amount of the tax, and :guilabel:`Applicable Code` defines if
-  the tax is to be applied. The formula is found at the bottom of the :guilabel:`Definition` tab.
-
-.. example::
-   :guilabel:`Python Code`: `result = price_unit * 0.10`
-   :guilabel:`Applicable Code`: `result = true`
-
 .. _taxes/active:
 
 Active
@@ -256,56 +378,112 @@ invoice line.
 Included in price
 ~~~~~~~~~~~~~~~~~
 
-With this option activated, the total (including the tax) equals the **sales price**.
+.. tip::
+   To set a company-wide default for this setting, go to :menuselection:`Accounting -->
+   Configuration --> Settings`, find the :guilabel:`Taxes` section, and set the :guilabel:`Prices`
+   setting to :guilabel:`Tax Excluded` or :guilabel:`Tax Included`. This setting cannot be changed
+   once invoices have been created.
+
+When set to :guilabel:`Default`, the company-wide default will be used.
 
-`Total = Sales Price = Computed Tax-Excluded price + Tax`
+When set to :guilabel:`Tax Excluded`, the tax will always treat the sales price on which it is
+applied as a price excluding the tax amount.
+
+When set to :guilabel:`Tax Included`, the tax will always treat the sales price on which it is
+applied as a total including the tax amount. The tax computation will split the sales price into a
+base amount and a tax amount. This makes it suitable for B2C sales in most countries, where prices
+are quoted tax-inclusive.
 
 .. example::
-   A product has a sales price of $1000, and we apply a *10% of Price* tax, which is *included in
-   the price*. We then have:
+   A product has a sales price of $1000, and we apply a 10% :guilabel:`Percentage of Price` tax
+   with :guilabel:`Included in Price` set to :guilabel:`Tax Included`. We then have:
 
    +-------------+-------------+----------+----------+
    | Product     | Price       | Tax      | Total    |
    | sales price | without tax |          |          |
    +=============+=============+==========+==========+
-   | 1,000       | 900.10      | 90.9     | 1,000.00 |
+   | 1,000       | 909.09      | 90.91    | 1,000.00 |
    +-------------+-------------+----------+----------+
 
 .. note::
-   If you need to define prices accurately, both tax-included and tax-excluded, please refer to the
-   following documentation: :doc:`taxes/B2B_B2C`.
-
-.. note::
-   By default, only the :guilabel:`Tax excluded` column is displayed on invoices. To display the
-   :guilabel:`Tax included` column, click the **dropdown toggle** button and check
-   :guilabel:`Tax incl.`.
-
-   .. image:: taxes/toggle-button.png
+   For a guide on configuring tax-excluded and tax-included prices for B2B and B2C customers,
+   see :doc:`taxes/B2B_B2C`.
 
 .. _taxes/base-subsequent:
 
 Affect base of subsequent taxes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-With this option, the total tax-included becomes the taxable basis for the other taxes applied to
-the same product.
-
-You can configure a new :ref:`group of taxes <taxes/computation>` to include this tax or add it
-directly to a product line.
-
-.. image:: taxes/subsequent-line.png
-   :alt: The eco-tax is taken into the basis of the 21% VAT tax
+If this setting is enabled, any subsequent tax applied on the same product line that has its
+:ref:`taxes/base-affected` will be based on a modified sales price. The exact behavior depends on
+the :ref:`Included in Price <taxes/included-in-price>` setting.
+
+.. tabs::
+   .. tab:: Tax-excluded
+      If :guilabel:`Included in Price` is set to :guilabel:`Tax Excluded`, subsequent taxes with
+      :guilabel:`Base affected by preceding taxes` will be based on a modified sales price equal to
+      the original sales price plus the tax amount.
+
+      .. example::
+         A product has a sales price of $1000, and we apply a 10% :guilabel:`Percentage of Price`
+         tax where the :guilabel:`Included in Price` setting is set to :guilabel:`Tax Excluded` and
+         the :guilabel:`Affect base of subsequent taxes` setting is enabled. Any subsequent tax with
+         its :guilabel:`Base affected by preceding taxes` will be based on a modified sales price of
+         $1100.
+
+   .. tab:: Tax-included
+      If :guilabel:`Included in Price` is set to :guilabel:`Tax Included`, subsequent taxes with
+      :guilabel:`Base affected by preceding taxes` will be based on a modified sales price equal to
+      the original sales price minus the tax amount.
+
+      .. example::
+         A product has a sales price of $1100, and we apply a 10% :guilabel:`Percentage of Price`
+         tax where the :guilabel:`Included in Price` setting is set to :guilabel:`Tax Included` and
+         the :guilabel:`Affect base of subsequent taxes` setting is enabled. Any subsequent tax with
+         its :guilabel:`Base affected by preceding taxes` will be based on a modified sales price of
+         $1000.
+
+This setting is considered any time multiple taxes are applied to the same product line, whether
+via a :ref:`group of taxes <taxes/computation>` or multiple taxes added directly to a product line.
 
 .. warning::
-   The order in which you add the taxes on a product line has no effect on how amounts are computed.
-   If you add taxes directly on a product line, only the tax sequence determines the order in which
-   they are applied.
+   The order in which taxes are applied depends only on the order in which they appear in the
+   :guilabel:`Taxes` list, not on the order in which they are added to a product line.
 
-   To reorder the sequence, go to :menuselection:`Accounting --> Configuration --> Taxes`, and drag
-   and drop the lines with the handles next to the tax names.
+   To modify the order, go to :menuselection:`Accounting --> Configuration --> Taxes`, and drag and
+   drop taxes using the handles to the left of the tax names.
 
    .. image:: taxes/list-sequence.png
-      :alt: The taxes' sequence in Odoo determines which tax is applied first
+      :alt: The order of appearance of taxes in the Taxes list determines which tax is applied first
+
+.. example::
+   In the following example:
+
+   - the Ecotax is a :guilabel:`Fixed` tax of €0.90 per unit, with the :guilabel:`Affect base of
+     subsequent taxes` setting enabled.
+   - The 21% VAT tax is a 21% :guilabel:`Percentage of Price` tax with the :guilabel:`Base affected
+     by preceding taxes` setting enabled.
+   - In the :guilabel:`Taxes` list, the 21% VAT tax comes after the Ecotax, as shown in the
+     configuration above.
+
+   When applying both taxes to a product line, the Ecotax amount is added to the basis of the 21%
+   VAT tax.
+
+   .. image:: taxes/subsequent-line.png
+      :alt: The Ecotax is added to the basis of the 21% VAT tax
+
+.. _taxes/base-affected:
+
+Base affected by preceding taxes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This setting, which is only visible in :doc:`developer mode <../../general/developer_mode>`,
+determines whether any previous tax that :ref:`affects the base of subsequent taxes
+<taxes/base-subsequent>` will modify the sales price that this tax is based on.
+
+.. note::
+   Taxes with :ref:`Included in Price <taxes/included-in-price>` always behave as if this setting is
+   enabled.
 
 Extra taxes
 ===========
@@ -320,7 +498,7 @@ imposed by governments. These extra taxes can be **luxury** taxes, **environment
 
 To compute an extra tax in Odoo, :ref:`create a tax <taxes/configuration>`, enter a tax name, select
 a :ref:`Tax Computation <taxes/configuration>`, set an :guilabel:`Amount`, and in the
-:guilabel:`Advanced Options` tab, check :guilabel:`Affect Base of Subsequent Taxes`. Then, drag and
+:guilabel:`Advanced Options` tab, enable :guilabel:`Affect Base of Subsequent Taxes`. Then, drag and
 drop the taxes in the :ref:`order they should be computed <taxes/base-subsequent>`.
 
 .. example::