For example, lets say I add Dr. Strange to a collection for Marvel Comics movies. This “Marvel” collection tag would be a custom one that I would have to manually add to every movie in it. Since Dr. Strange does not currently have a collection of TMDb, the “Marvel” collection tag is the only one it would have on my server. But maybe in a couple years they will decide to make another movie called Dr. Strange 2. TMDb now has a collection for Dr. Strange because there are 2 movies. After adding the second movie to my collection, my Plex server would add the “Dr. Strange” collection tag automatically. But the first one would not be automatically added to that collection because I have used a custom “Marvel” collection tag and the tag field would be locked. So this means I would have to go back and unlock the tag field and refresh the metadata. Then I would have to add the custom tag again since it would have been overwritten during the refresh due to the field being unlocked. While this task isn’t that long, it would be a lot of work to maintain all the tags for a large movie server with several custom collections.
If there were a collection tag field reserved for TMDb or for whichever agents you’re using, I think this problem could be eliminated. So you would end up having agent collection tags and custom collection tags.