I like to add additional tags via Discogs, and I imagine many others do too. The field is typically called DISCOGS_RELEASE_ID. Musicbrainz already trims the URL of the name, so the mapping would just have to delete "https://www.discogs.com/release/." Plus, this would let me know at a glance which albums in my library don't have a Discogs URL on MB, so they can be added too.