In an immediate-or-cancel order, the order is filled immediately at the best offer price for buys and best bid price for sells at that moment. They allow brokers to fill large orders at the same price, leaving nothing outstanding in case of price fluctuations. So, if an immediate-or-cancel order can only fill half of its need, the number of shares remaining to be filled is simply canceled.
This is different from a good-til-canceled order, which will persist until it's filled at a set price point.
Benefits of a good-til-canceled order
A good-til-canceled order is a reliable way to trade all the shares of a stock that you want without paying more or taking less than you want. Because you can set the threshold at which your stocks are bought or sold, you also don't need to watch for market movement since the computer will do all of that for you and react accordingly.
Being good-til-canceled also means that the trade goes on and on, even when you're working on something else. You may anticipate a drop in a stock price, but you're not sure when it's going to happen, so you place a good-til-canceled order for the next 90 days to catch that anticipated, but not precisely predictable, dip.
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