That's the official term for the legalised 'apartment girls' set up.
I want to report that I've found a bunch of these in the heart of Causeway Bay. Get a cab or an MTR to the New York Cinema, in Causeway Bay Plaza 2. Opposite the cinema, near the 7-11 at 474-476 Lockhart Road is a staircase leading up to an old residential building. On the 6th and 7th floors (probably the others too) are lots of apartment doors with the usual "welcome/ please wait" signs hanging outside. I can report that the girl currently in 7th floor apartment H is HOT and very eager to please.
I like this set up because it is less likely to be raided than a Mongkok brothel, is cleaner and there's no brothel operator waiting to take his $200 cut. They are also cleaner and newer than Mongkok places, because the girls handle their own tidyup. Without so much skimming from the gangsters and as the girls don't have to prowl around in beauty parades looking for business, prices should be cheaper than Mongkok. I believe prices range from $200 to maybe $350 (about US$40).
So, now you can get laid for less than the price of a martini and a tab of rohypnol and still not have to cross the harbour.
Let's use this thread to compile locations and building addresses that contain these businesses.