Enhance offline-first documentation on synchronization

content/en/docs/refguide8/mobile/offline-first.md

@@ -111,6 +111,17 @@ If you have custom widgets or JavaScript actions which use an entity that cannot
 
 For each entity the **Synchronization mode** is shown. A default is automatically determined by analyzing the model, but can be overridden by the developer in which case the setting will appear in boldface.
 
+When an entity is configured with the Online, Nothing (clear data), Nothing (preserve data), or Never synchronization mode, the Mendix runtime will reject any synchronization request from a client that attempts to upload changes for that entity. This prevents unintended data modifications on the server and makes your application more secure.                                                                                                                                         
+If a client attempts to synchronize changes for a restricted entity, those changes are blocked and a synchronization error is recorded on the server. You can inspect these errors via the System.SynchronizationError entity.                                                                                                                                                                                                                        
+**Compatibility Mode** - When you migrate an entity from All Objects or By XPath to Online, Nothing, or Never, older versions of your offline app may still attempt to synchronize changes for that entity. To prevent those clients from receiving synchronization errors during the migration period, you can enable Compatibility Mode for the entity by selecting the checkbox in the synchronization configuration.                                                                                                  
+
+When Compatibility Mode is enabled for an entity:
+* The runtime accepts synchronization requests from clients for that entity, regardless of the configured synchronization mode.
+* The entity behaves as if it were still set to All Objects.
+
+When you are confident that all old clients have updated to the latest version, you should disable Compatibility Mode by deselecting the checkbox.                                                                                                                                                                                                     
+When you change an entity's synchronization mode to Online, Nothing, or Never in Studio Pro, Compatibility Mode is automatically enabled to avoid breaking existing clients.
+
 ### Limitations
 
 Running multiple synchronization processes at the same time is not supported, regardless the of the type (**full** or **selective**). For more information, see the [Limitations](/refguide8/synchronize/#limitations) section of the *Synchronize Reference Guide*.