Update RUM sampling guide to account for RWOL

content/en/real_user_monitoring/guide/best-practices-for-rum-sampling.md

@@ -11,57 +11,90 @@
 
 Sampling in Datadog's Real User Monitoring product enables you to collect data from a certain percentage of user traffic.
 
-This guide walks you through best practices for RUM sampling so you can capture sessions and collect data based on your monitoring needs. Learn more about how [sessions are defined][9] in RUM.
+There are two different ways of sampling, which control the data you send to Datadog:
 
-## Sampling configuration
+- **Client-side (head-based) sampling**: Makes the sampling decision at the beginning of a user session, before any data is collected. The RUM SDK in your application determines whether to track the entire session or not, reducing data collection and ingestion of sessions that aren't analyzed.
+
+- **Server-side (tail-based) sampling**: Makes the sampling decision after data has been collected and sent to Datadog. It allows you to filter and retain specific sessions based on their characteristics (like errors or user attributes) using retention filters.
 
-### Configure the variable names that correspond with the SDK version
+  **Note**: Server-side sampling is only possible with the [retention filters][12] provided by [RUM without Limits][2]. If you need to use this but are on the legacy, client-side-only model, reach out to your account team.
 
-Sessions are randomly sampled based on the percentage listed in the [SDK configuration][1]. To that end, make sure to use the correct configuration variable names for the SDK version being used.
+This guide walks you through best practices for RUM sampling so you can capture sessions and collect data based on your monitoring needs. Learn more about how [sessions are defined][1] in RUM.
+
+## Sampling configuration
 
 ### Configure the sampling rate
+
+#### Client-side (head-based) sampling rate
+
+With [RUM without Limits][2], client-side sampling rate helps you control how many sessions you send from your applications to Datadog.
+
 Before each new user session, the SDK draws a random floating-point number between 0 and 100, which is then compared to the value set in the SDK configuration. If the random number is lower than the value set in the SDK configuration, the session is kept and events start being collected. If the random number is higher, the session is not kept and events are not collected until the end of the session.
 
-You can set the sampling rate with the SDK ([Browser][2], [Android][3], [iOS][4], [React Native][5], [Flutter][6], [Roku][7]), then deploy it in the application code.
+You can set the sampling rate with the SDK ([Browser][3], [Android][4], [iOS][5], [Flutter][6], [Kotlin Multiplatform][7], [React Native][8], [Roku][9], [Unity][10]), then deploy it in the application code.
+
+#### Server-side (tail-based) sampling rate
 
-Only sessions that are sampled are available in RUM. For example, if the sampling rate is set to 60%, then 60% of all sessions and metrics (such as Core Web Vitals and usage numbers) are visible in RUM.
+With RUM without Limits, server-side sampling rate defines which sessions you want to keep in Datadog (see details about the [retention period][11]).
 
-The random sampling is by session, not by user.
+The server-side sampling rate is defined as part of the retention filters for your sessions. When a retention filter matches a session or matches one of the events making up the sessions (view/action/error/resource, and so on), the whole session is stored alongside all its events (and including the ones that preceded the sampling decision). The retention rate allows you to store only a specific percentage of sessions that meet the filter criteria and discard the rest. Learn more about [how retention filters work][12].
 
 ### The effect of sampling on data and metrics that are available in RUM
-RUM metrics (such as Core Web Vitals and usage numbers) are calculated based on sessions that are sampled. For example, if the sampling rate is set to capture 60% of sessions, then the Core Web Vitals and total number of sessions are calculated based on 60% of those sessions. 
+
+RUM metrics, including the ones that come [out-of-the-box with RUM without Limits][13] (such as Core Web Vitals and usage numbers) and the [custom ones][16] that you can create yourself, are all calculated based on sessions that are ingested on Datadog. For example, if the client-sampling rate is set to capture 60% of sessions, then the Core Web Vitals and total number of sessions are calculated based on 60% of those sessions.
+
+**Note**: With RUM without Limits, those metrics are computed before the [retention filters][12] - in other words, before server-side sampling.
 
 ### Recommended sampling rate
-In terms of setting an ideal sampling rate, it depends on the amount of traffic you see and the data you are looking for. Datadog recommends starting with a sampling rate you are comfortable with based on your budget and estimated traffic, then tweaking it based on the data you need. 
 
-### Sampling based on specific attributes
-Configuring sampling based on specific attributes, such as sampling 100% of sessions with errors and 5% otherwise, or only sampling sessions that go through the checkout flow, is not supported. If this feature is critical for your business needs, create a ticket with [Datadog Support][8].
+#### Client-side (head-based) sampling rate
 
-### Changing the sampling rate in the Datadog RUM UI
-Changing the sampling rate in the Datadog RUM UI is not supported. If this feature is critical for your business needs, contact [Datadog Support][8] to create a feature request.
+For optimal monitoring, Datadog recommends sending 100% of your sessions to Datadog. This ensures accurate out-of-the-box custom metrics, and complete visibility into your user experience.
+
+However, if your application experiences high traffic and ingestion costs are a concern, you can reduce the sampling rate. Keep in mind that lower sampling rates affect the accuracy of your [metrics][13] proportionally.
+
+#### Server-side (tail-based) sampling rate
+
+For server-side sampling, Datadog recommends a two-step approach:
 
-### Adjusting sampling during live outages
+1. Start with basic retention filters to capture sessions with critical user paths, such as errors or from specific users.
 
-If a bug or incident occurs, you can increase sampling to collect 100% of your browser user traffic to ensure no session is missed. You need to deploy a code change to achieve this.
+2. Adjust the sampling rate based on your needs:
+   - Ensure you have enough sessions for troubleshooting
+   - Maintain sufficient data for APM correlation
+   - Keep enough samples for performance analysis (waterfall views, long tasks)
 
-**Note**: This capability does not apply to mobile or Roku applications due to the long release cycle.
+With RUM without Limits, your server-side sampling should provide enough data for both troubleshooting and performance analysis while managing your data volume effectively.
+
+### Sampling based on specific attributes
+
+Configuring sampling based on specific attributes, such as sampling 100% of sessions with errors and 5% otherwise, or only sampling sessions that go through the checkout flow, is supported using [retention filters][12]. See the [Retention Filters Best Practices][14] guide to understand common retention filter types.
+
+### Changing the sampling rate in the Datadog RUM UI
 
-### Accounting for mobile devices that go offline or crash
+Changing the sampling rate is only supported for server-side sampling, and can be accomplished by [modifying the retention rate][15] from the retention filters page.
 
-RUM ensures availability of data when user devices are offline. In low-network areas, or when the device battery is too low, all RUM events are first stored on the local device in batches. They are sent as soon as the network becomes available, and the battery is high enough to ensure the RUM SDK does not impact the end user's experience. If the network is not available while your application is in the foreground, or if an upload of data fails, the batch is kept until it can be sent successfully.
+During live outages, incidents, or bug investigations, you can increase sampling to collect 100% of your sessions to ensure nothing is missed, or to have more examples of a particular issue.
 
 **Note**: This behavior only applies to mobile applications using mobile RUM SDKs.
 
 ## Further Reading
 
 {{< partial name="whats-next/whats-next.html" >}}
 
-[1]: /real_user_monitoring/guide/sampling-browser-plans/#overview
-[2]: /real_user_monitoring/guide/sampling-browser-plans/#overview
-[3]: /real_user_monitoring/mobile_and_tv_monitoring/android/advanced_configuration/?tab=kotlin#initialization-parameters
-[4]: /real_user_monitoring/ios/advanced_configuration/?tab=swift#sample-rum-sessions
-[5]: /real_user_monitoring/reactnative/#initialize-the-library-with-application-context
-[6]: /real_user_monitoring/mobile_and_tv_monitoring/flutter/setup/advanced_configuration/#sample-rum-sessions
-[7]: /real_user_monitoring/mobile_and_tv_monitoring/roku/setup/#initialize-the-library
-[8]: /help
-[9]: /real_user_monitoring/guide/understanding-the-rum-event-hierarchy/#sessions
+[1]: /real_user_monitoring/guide/understanding-the-rum-event-hierarchy/#sessions
+[2]: /real_user_monitoring/rum_without_limits/
+[3]: /real_user_monitoring/guide/sampling-browser-plans/#overview
+[4]: /real_user_monitoring/mobile_and_tv_monitoring/android/advanced_configuration/?tab=kotlin#initialization-parameters
+[5]: /real_user_monitoring/mobile_and_tv_monitoring/ios/setup/#sample-session-rates
+[6]: /real_user_monitoring/mobile_and_tv_monitoring/flutter/setup/#sample-session-rates
+[7]: /real_user_monitoring/mobile_and_tv_monitoring/kotlin_multiplatform/setup/?tab=rum#sample-rum-sessions
+[8]: /real_user_monitoring/reactnative/#initialize-the-library-with-application-context
+[9]: /real_user_monitoring/mobile_and_tv_monitoring/roku/setup/#step-3---initialize-the-library
+[10]: /real_user_monitoring/mobile_and_tv_monitoring/unity/setup#sample-rum-sessions
+[11]: /data_security/data_retention_periods/
+[12]: /real_user_monitoring/rum_without_limits/retention_filters
+[13]: /real_user_monitoring/rum_without_limits/metrics
+[14]: /real_user_monitoring/guide/retention_filter_best_practices/
+[15]: /real_user_monitoring/rum_without_limits/retention_filters#modifying-filters
+[16]: /real_user_monitoring/platform/generate_metrics