Currently the only relevant release types are Official and Promo. This is not really sufficient to incorporate pre-releases / standalone track releases that are not singles / shop bonuses with the way Official Release Groups work.

I've thought it cleaner to store pre-release single track excerpts of an album in the same release group as the album. However, this probably breaks the concept of "original release date" and also creates 1-track Albums when it's really a pre-release "single" of no discographic importance (it is an Official release, not a Promo release, which no one wants to see a standalone Release Group for.)

Shop bonuses ("attachments") are a similar issue.

I have two ideas on how to handle this:
1. Add "Pre-releases" and "Bonus" Release types so that these kinds of releases can still be stored in the same Release Group as the full release. Since they are no longer Official release types, things like "original release date" can ignore the attributes of these releases.
2. Create an "Attachment" release group type and a Release Group-Release Group relationship to link an "Attachment" to its actual release group. Release groups containing only Attachments, like with release groups containing only Promos, would then not show up on the default artist overview. However, this doesn't logically fit in with how multiple standalone tracks may be released simultaneously from an album, and definitely has a relationship cluster problem (3 standalone attachment release groups which are all technically the same "group").

I like #1 way more than #2 but don't know how others feel. There has previously been editor/voter resistance to calling pre-release tracks from an album "Singles", as well as resistance to having them in a standalone "Other" release group.