Besides the time you submitted the order, are there any other determinants of priotity? For example, does a 50 contract limit order get more priority than a 1 contract limit order? What if it's a 50 contract all-or-none? Say you have a 50 contract limit all-or-none resting on the bid. There are 5 contracts in front of you. Now a market order to sell 50 contracts comes in. Who gets filled?