many customers share a common fetish, they love the small patch of skin peeking out between the girl's thighs just below the skirt, and right above the over-the-knee socks. they call this part of a woman the "absolute territory" (or "domain", 絶対領域 "zettai ryouiki")
it appears that this "absolute territory" thing gets its origin from a famous manga tale. its exact definition has to do with the proportions of the costume. for the diehard fetishist, the ratios in length of a skirt, the thigh, and the upper parts of the over-the-knee socks must be 4:1:2.5. apparently, all the maid costumes are based on this "absolute domain" proportion.
(for some reason, it reminds me of the "sectio aurea" in architecture and, on a funnily unrelated theme, of the "Reinheitsgebot" German law about beer making...)