The Accessory Instance is the parent Instance of all accessories (regardless of their specific accessory type). It typically has a child Handle with a child Attachment and a WrapLayer in the case of Layered Clothing.
The Accessory class is the successor to the legacy Hat system. It's cross-compatible with both the legacy R6 character system and the new R15 character system.
If you insert an Attachment into the Accessory's Handle with the same name as an Attachment in one of the character's limbs, they connect and ignore properties inherited from the Accoutrement class. Otherwise, the Accessory functions identically to a Hat.
Note: If there are two matching Attachments, the resulting Weld is a child of the Accessory's Handle. This differs from the legacy behavior of Hats where the Weld is always a child of the Head of the character.