@bflaminio: Nice clarification. I agree that the existing ‘compilation’ type could be used to embrace the three instances (after an update of the guideline). The two first instances can already be distinguished from the release/RG artist (The first is from Various Artists, the second is more specific.) and thus do not require to be distinguished by a MB specific term. I was gradually focused on the ‘anthology’ term and partly neglected the initial subject which is the third instance that overlaps the two others. This last type of instance can be split further as follows:
3.a. A collection of previously released albums, using the same packaging down to the barcode and other details, bundled in an over-packaging (then an entity-release relationship is useful as it adds data which could not be retrieved from the database otherwise)
3.b. A collection of previously released albums in a custom packaging (then an entity-release group relationship is useful, but an entity-release one would not as it does not add data which could not be programmatically retrieved from existing disc ID, tracklist, or shared recordings)
As for the source entity of the above (to-be-invented) relationships, it primarily is a release group, but it can additionally be a release when variations exist (and only then).
@psychoadept: I don’t deny that the term is widespread in catalogues, I don't take the Amazon US definition or the WP one for official/only valuable definitions, and I agree that it introduces confusion. This is why I don’t think it is a good idea to use this term into the MBz classification system as an RG type (since it is confusing from the context of RG types) but potentially as an (over)packaging type (if it is clear from the list of packaging types).
@tommycrock: The more I dig the database, the more I find contradictory examples, like “Glam Rock Anthology” which is from different artists and “Compilation” which is from a single artist. My distinction between the two terms is clearly not shared by everyone, not to say artificial.