docs: add callout explaining sessions vs session replay distinction

Author: inkeep[bot]

contents/docs/data/sessions.mdx

@@ -24,6 +24,17 @@ A session is a set of events that try to capture a single use of your product or
 
 Sessions enable you to analyze how exactly users are using your product, their entry and exit points, how long they spend on your site, how active they are, bounce rate, and more.
 
+<CalloutBox icon="IconInfo" title="Sessions vs Session Replay" type="fyi">
+
+**Sessions** and **Session Replay** are related but distinct concepts:
+
+- **Sessions** - An analytics concept where events are grouped by `$session_id` to represent a single visit. Used for metrics like session duration, bounce rate, and entry/exit paths.
+- **Session Replay** - A visual recording feature that captures what users actually did during a session, including DOM changes, mouse movements, clicks, and scrolls.
+
+Not every session has a recording. Session Replay must be [enabled separately](/docs/session-replay/installation) and has its own [billing limits](/docs/session-replay/how-to-control-which-sessions-you-record#billing-limits). You can use session analytics without Session Replay, and vice versa.
+
+</CalloutBox>
+
 ## How does PostHog define a session?
 
 Our [JavaScript Web library](/docs/libraries/js) and mobile SDKs (Android, iOS, React Native, and Flutter) add a `$session_id` property to each event. Events captured from the same user, browser, and device get the same `$session_id` within the same session, until a new session is started. A new session is started by default when either there's **no activity for 30 minutes** or the session has reached the maximum session duration of 24 hours. Subsequent events are grouped into a new session and a new session triggers a new session replay.