I think he means various models meant as attachments.
Usually you use an aura I think - like Devotion Aura, that affects no unit, has no effect, etc. but instead of Devotion Aura visual effect (the rune on the ground), you put the hat model there and set it's attachment point to head.
Or you do it via trigger editor by attaching special effects to unit - that way you don't have to create millions of auras for various attachments.