When upgrading Aurea Messenger (aka CX Messenger / Sonic) for eg. to SP18 (10.0.18.169 from 10.0.11.126) you would like to know whether it is necessary for it to be compatible with the Aurea Monitor (Actional / Intermediary). You would like to know whether do you need to also upgrade to Aurea Monitor v11 or v12.
Only the version of Aurea Monitor SDK used by Aurea Messenger needs to match the version of Aurea Monitor used with Aurea Messenger.
You don't need to use matching versions of Messenger (Sonic) and Monitor (Actional).
Aurea Messenger ships with a certain version of actional-sdk.jar and actional-plugmaker.jar files. The version of these jar files has to match the version of the Aurea Monitor Agent.
For eg: If you use a version 12.0 Monitor Agent you need to update the mentioned jar files used by Messenger to match that version. Since Messenger SP18 ships with version 11.x of the jar files, in this case, this means that the mentioned jar files need to be updated to version 12.x. If you would be using version 10.0 of the Monitor Agent, then you would need to "downgrade" the jar files to use the ones from version 10.x
Read the article How to update the Actional SDK jar files in Sonic to know how to update the Monitor SDK jar files used in Messenger.