This allows you to pass in a custom id for your own reporting purposes. This ID will be present in the new Video SDK APIs and Webhooks.
Correct. There is also a in session userId that Zoom generates that identifies the user so the developer can call the other Video SDK in session APIs for that user.
Example, the stream.muteAudio(userId?: number) function.
The one in the signature is for the Video SDK APIs and Webhooks, the one that Zoom generates for the live session is for the in session functions.