I just remembered there’s a test server for just this kind of thing.
Annotations are combined, as reosarevok thought. The annotation from the target release comes first, followed by a horizontal line and then the source annotation.
Artist credits that are identical except for one having a date and the other no date, are merged into a single credit that includes the date.
Artist credits that differ by instrument “credited as” are not merged. So if one release had “guitar: Jimi Hendrix” and the other had “guitar [lead guitar]: Jimi Hendrix”, both will appear as separate relationships on the merged recording.
Artist credits are merged when one has the artist “credited as” specified and one does not. So credits for “guitar: Jimi Hendrix” and “guitar: James “Jimi” Hendrix” would result in a single credit with the as-credited name. If both source and target have ‘credited as’, the as-credited name from the target is used.