Excuse me if it is in the manual (RTFM anyone? ;-))
Of course, it's in the manual but, this time you are excused, because we updated the manual after 2.9.5, as a Hotfix, without increasing the version, so you might have not got it or not noticed it's there.
So, just run the Live Update now, so you'll be surely getting the very latest manual and, at Page 81, there's a complete chapter dedicated exactly to explaining how the Variable number of Luggage/Cargo works (it does) and how the Cargo on Simbrief doesn't affect the number of carts on Passengers planes.
Now, this is only related to the variable NUMBER of carts.
The issue with the carts TYPE is a completely different matter and, it seems the issue here is that Fenix made a bit of mishap in their GSX profile: they set the Loader type to AUTO, which doesn't mean "set the correct type automatically" but, instead, means "set EITHER ULDs or BELT loaders depending on the cargo door's HEIGHT" so, on an A320, it will always result in BELT loaders no matter what, because its cargo door is lower than the "AUTO" threshold.
On a custom airplane profile, you usually set the loader Explicitly, either BELT or ULD, which for an A320 is supposed to be AKH only.
The loader TYPE and the meaning of the "AUTO" setting hasn't been changed in a long while.