There are a variety of things which are not available as standalone releases and are only available as part of a larger product, where the larger product is something not relevant to us. We often use "promotion" for these because they're sort of free and sometimes they're used as a marketing tool ("buy this magazine and you'll get a CD too!") but quite often that seems a bit weird to me because they're usually only free if you buy something else to get it and it's not always the case that it's used as a marketing tool either (e.g. books for learning languages often come with a CD, but that's not really used to try and convince people they want to buy the book but as a way of providing relevant audio).

VGMdb uses "Enclosure" for these, which it describes as "Catch-all type for albums that were never commercially sold as standalone releases, but distributed as part of a goods package (such as a videogame or magazine). Note that enclosure albums sometimes have their own catalogue number."

Examples:

https://musicbrainz.org/release-group/c3bbcd57-4374-3e1f-a2d5-35369b314eb0 - While the Japanese release was a normal single, the US release was only available as part of one of the TV series' DVD releases.

https://musicbrainz.org/release/f613c7ce-afc5-4cde-8b8f-c5838bfe31ea is a CD that comes with a language textbook.

https://musicbrainz.org/release/a382a38f-51ff-4075-84c4-edbf57928fd1 apparently came with the Japanese game.

https://musicbrainz.org/release/c1342e25-f3a8-44e2-aa9b-14bb893f0260 came with a magazine.

I'm not sure if "enclosure" is the best name, but I can't think of anything else that would work.