That, or they could explain it better in-game, or something.

I'm not suggesting a nerf, it just sounds weird the way they worded it, and nothing in game CALLS them that.

"Advanced Materia Slot" ... and I'm looking at my items and all the materia slots an item has look the same. So the logic goes "Okay so all materia slots on all items look the same, maybe they're all called that?"

Maybe what they could do, is for example an item has 1 materia slot... they could put the 1 materia slot there, and then put 1 red materia slot there to denote that one Can take an overmelded materia?

Or, yes, "Can only be overmelded once per item" would work too.