helraiser, basically means a stinky hole: you can buy and sell of course, but you have no reference to look at: no bid, no ask, no L2... only matched trades: if you are lucky and your price of buy/sell matches with the price of a sell/buy good for you... if not, you can surprisingly see how sells with a PPS well above your sell order goes thru as well as buys well below the PPS of your buy order.
Hope this helps. GLTY!