An Immediate or Cancel (IOC) order allows a Trading Member to buy or sell a security as soon as the order is released into the market, failing which the order will be removed from the market. Partial match is possible for the order, and the unmatched portion of the order is canceled immediately. The IOC order is used very often by algorithmic traders who do not want to be seen waiting passively in the system so most of their orders are IOC orders. If the order is not executed immediately it is automatically canceled. That is why you will find that in case of such algorithmic system orders, you will find a very high ratio of orders to trades. Most of the orders are IOC orders which are automatically programmed to cancel if not executed immediately.