Use the createSession command in the playbackSession namespace to unconditionally create a new session and clobber any existing sessions.
Use appContext to determine how multiple instances of your app can share control of a session. For example, if you provide a user account identifier as appContext, then two instances of your app logged into the same user account would be able to control the same session on a group. If you choose to implement your app to always provide an appContext that is unique for all app instances, then only one app instance can control a session at any time.
Use customData to save information in the session that your app finds useful. For example, your app could store a playlist identifier in customData, so that another app instance could automatically load the right playlist when joining a session.