The marks on her collar resemble the mathematical symbols ∴ (therefore) and ∵ (because), leading to the theory that they represent her "Ability to Manipulate Fate"

"S" or "6" shaped brooch. Fan art often depicts one of the below decorations. The first, aristocratic-looking one is apparently called a "Jabot". The second is a necktie-like ribbon to match Flandre.

Bat wings are evolved from palms. The upper protrusions correspond to human thumbs.

Though hidden and unseen, it looks like she wears a suspender skirt.

A made-to-order garment with slits in the back to let her wings out.

Reference: The Great Touhou Project Exhibition Bats are so interesting! Their ecology and conservation methods are surprisingly unknown.

※Contains multiple headcanons and points of speculation