An Auction order is entered into the electronic trading system during the pre-market opening period for execution at the Calculated Opening Price (COP). If your order is not filled on the open, the order is re-submitted as a limit order with the limit price set to the COP or the best bid/ask after the market opens.
An Discretionary order is a limit order submitted with a hidden, specified 'discretionary' amount off the limit price which may be used to increase the price range over which the limit order is eligible to execute. The market sees only the limit price.
A Market order is an order to buy or sell at the market bid or offer price. A market order may increase the likelihood of a fill and the speed of execution, but unlike the Limit order a Market order provides no price protection and may fill at a price far lower/higher than the current displayed bid/ask.
A Market If Touched (MIT) is an order to buy (or sell) a contract below (or above) the market. Its purpose is to take advantage of sudden or unexpected changes in share or other prices and provides investors with a trigger price to set an order in motion. Investors may be waiting for excessive strength (or weakness) to cease, which might be represented by a specific price point. MIT orders can be used to determine whether or not to enter the market once a specific price level has been achieved. This order is held in the system until the trigger price is touched, and is then submitted as a market order. An MIT order is similar to a stop order, except that an MIT sell order is placed above the current market price, and a stop sell order is placed below