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!
The result of all our acts in life is the reward obtained from the equation between risk (luck) and sureness (knowledge). That´s why I love Markets!