Sadly, both not really true. A Tag is not static, for the reasons that @AndersVinberg mentions. It is not “user-defined” because Roon choses other stuff to put “in the tag”, which the user did not ask for.
I moaned about this in some detail in What happened to my Tagged Compositions?. I find this to be pretty much the only thing the Roon team has managed to screw up since release. And as Tags are a minority feature, not enough people complain about it for it to get high enough up the list. So those of us who need tags end up dealing with a system which functions poorly.