It's all about having an exposed barrel tip. If the holster completely covers the tip of the barrel and is fully closed (and hopefully reinforced), then you can usually get away with calling it a "barrel bag"... that happens to be strapped to your belt.
Otherwise you bag it like a normal gun.
Now some older pistol, like the classic PGP, are setup in such a way that most normal barrel bags won't fit. I've gotten special permission to use an old school plug on mine a few times, once I demonstrated that they properly fit. Though I will be finding an oversized barrel bag for mine for next season.