The life of a musician is fun and friendly. I like the fact that they can be free to express themselves however they wish to. The session musician is like someone who joins a crew for a particular event or stands as a backup singer or a musician but he is not part of the crew.
The session musician is mainly for a particular event or season, he is not the main artist he basically supportd or adds vibes to the team. I will simply say he is a contract singer or musician.