How a good-til-canceled order works
Here's an example of how a good-til-canceled order works:
You decide that you want to buy 5,000 shares of ABC, Inc. for $17.50 per share or less and place a 90-day good-til-canceled order. On day one, ABC, Inc. is at $19 per share, so nothing happens. It remains around this level for a week, and then on day nine, it drops to $16 per share unexpectedly.
This is when your good-til-canceled order kicks in. The moment the computer detects the shares dropping in price, it pounces, buying up as many shares as it can below your specified threshold. But, in this case, there were only 1,000 shares to be had.
That's OK; your good-til-canceled order is still good because it wasn't canceled. It's just now only looking for 4,000 shares. This pattern repeats itself until your share order is filled or other cancellation conditions are met.
Good-til-canceled orders also work in reverse, if you want to sell 5,000 shares of ABC, Inc. Instead of your order waiting for a price below your specification, it looks for a price above your specification and sells as many shares as it can at that price until they're gone.