we have "Auto" as main category
"Audi" is subcategory

Only moment i select "Audi" when i create an ad, see the options "Cars"
"Parts"
"Accesories"

if i click on "Parts", only than see "Doors", "Hood" and so on.

all this must be specific per category.

i tried with multifield plugin ad didn't work as i wanted so, is it posible?