I believe the Release Disambiguation field is serving two different purposes, and might benefit from being split up into two different fields, e.g. Disambiguation and Subtitle.

One of these uses seems to be to disambiguate things for people adding and editing Releases, so they make sure they're working on the right one. "Made in Germany by Warner Manufacturing Europe" and "reissue, 28 tracks. "Whoops Now" on separate track 28" would fall into this category. They help contributing users to make sure they're editing the right Release, but you don't want them showing up in the metadata that gets saved with the music files in your library. They're also not explicitly named in the packaging.

Another use seems to be to disambiguate releases within people's actual music libraries (completionist type people, at least). They're not just of interest to people editing the metadata, but also useful to people using that data to sort their library out. "30th anniversary edition" and "CD1" would fall into this category. You can have CD 1 and CD 2 of a pair of singles, and you can have an original album and also its deluxe edition, and these keep them separate in your music player. If not a fully fledged subtitle, they're at least a main selling point. They are usually explicitly named in the packaging.

This is not to be confused with the Release Group Disambiguation, which I believe is sufficient as it is.

(See also the forum thread on this.)