A release group is a collection of releases, not a collection of recordings. A release group is an album, in all its forms. The release is the specific instance of that album, released on a certain format, on a certain date, in a certain country. The recording is a particular instance of a song, so a studio version of a song would be a different recording from the live version of the same song. A work can be thought of as the “sheet music” of a song and will connect to the different recordings.
For example: Uprising by Bob Marley and the Wailers. Inside that release group, you’ll find releases like the 1980 French vinyl release or the 1992 British CD release. Then on those releases, you’ll find the tracks that link to the recordings of Redemption Song.