The session attribute is usually paired with the user attribute. A Session object has information pertaining to a particular user session through which the activity was initiated. User is an entity object that isn’t always associated with a session, and isn’t always an actor, hence Session isn’t part of the User object, but is included with the Actor object for actor semantics.
Related to this, the process attribute of type Process has a User object which represents the user’s account that the process runs under or is impersonating. Hence, the Process object also has a session attribute paired with its user attribute.
Often, User and Session objects will be paired in many event classes.