Is this a conforming item or dynamic? If dynamic, you'll want to vertex/poly group in addition to material grouping. And you'll want to make the mesh contiguous (all one piece) or it will fall apart in the sim.

For conforming you don't have to worry about it. If you plan on making a fair bit of clothing, I'd suggest you get your head around rigging - agreeing with pjz99 from other threads: it's worth the time investment - otherwise, get a copy of Obj2Cr2 and rig with that. Easier, but you have much less fine control over how your item is rigged. No offence, PhilC: just nothing beats doing it manually. I've finally pulled the finger out and have rigged my first clothing item, and yes, it's not easy but no, it's not way too difficult either... well worth-while learning. Go with Phantom3D's rigging tutorial: it will demystify the whole process!

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks