As suggested in https://chatlogs.metabrainz.org/brainzbot/metabrainz/msg/4350387/

for a recording, concert (event) or full album, there can be artists who are "session" musicians, today these might be stored as "guest" but that's not quite the same thing.
A guest Musician is often someone who's billed specially, in the vein of "featuring" perhaps.

A session musician is often a session musician by vocation (or established musicians can be moonlighting as session musicians, esp early in their career)

Example: https://beta.musicbrainz.org/release/621666f8-dc7e-3d04-8359-562dff7dd0d4/edit-relationships (Simon Phillips is actually a session drummer, not a guest drummer)