Most CDs have a “release catalog number” usually found on the spine. There are a lot of record companies / manufacturers who have an additional concept of a “medium catalog number”, which is sometimes simply a “1” or “2” (the disc number) tacked onto the end of the release catalog number. e.g., http://musicbrainz.org/release/b4e8f7f4-e35c-42eb-a043-9605e4784aff
Sometimes the medium catalog number is numerically part of an entirely different sequence. e.g., http://musicbrainz.org/release/947114e7-1916-407d-9770-44369e8dd3cf
I don’t think these medium catalog numbers belong in the release catalog number field because they are usually NOT a way to lookup a release in stores. (e.g., for releases in Japan, the release catalog number is really significant - some stores may offer lookup via release catalog number instead of via barcode, but if you have a mix of release and medium codes, you would have to be intimately familiar with the record company to know which code to use). I’d rather they just go in the annotation so it is clear the code does not represent a way to lookup the release.
How do others feel about this? It would be nice to get consistency via written Style.