Recording sessions are just the base, the frame of any artist’s discography.
A recording session, a live concert, a TV/Radio/Video appearance group (include) several recordings made by artist at the same location/studio, in the same date, with the same staff: musicians, recording engineer, producer. (So far we should add these relationships separately for each recording from a session/concert.)
Sometimes a release includes just a recording session (or some sessions) or, very often, a live concert or several appearances. So, the Release Group can have a relationship “Includes session” with the Session.
But so far this very important concept is absent in MB.
I suggest to introduce the “session/concert/appearance” notion in MB as “Session”.