Pre-NGS, there was no release groups, only releases.
All editions with same tracklist were forming a single release with several release events (one per edition).
A release event was: one date, one country, one label, one catalogue number, one barcode.
So, different Disc ID for all those editions were added to this unique release.
Usually most of those edits would be without edit notes telling for which edition (release event) this Disc ID was.
In NGS (now) we have release groups and each release is only one single edition.
For releases added in NGS, there should be only one Disc ID (it’s extremely rare to find distinct Disc ID on a physically identical release).
But the migration of pre-NGS releases to NGS could not guess this Disc ID is for this or that edition (edit notes telling so were extremely rare, and anyway edit notes are free text).
So, when release events were migrated to split releases (those we now know), one release for each release event, and each release ended up with all the Disc ID from the pre-NGS release (all editions).
Now it is very welcome to cleanup and remove redundant unidentified Disc ID or to keep them in a vague catch-all release and adding well defined releases with unique Disc ID.
But there is a text displayed on the Disc ID removal page that seems to come from pre-NGS and that strongly discourage from removing any Disc ID:
You need to be certain that this disc ID was added to this release erroneously, since a release can have multiple valid disc IDs, and each disc ID can belong to more than one release. For more in-depth information about this topic, please see our CD submission guide.
This part “a release can have multiple valid disc IDs” was absolutely true pre-NGS but I don’t think it’s correct to say that today.
I feel the whole paragraph could be removed…