A Good-Til-Cancelled (GTC) order is an order to buy or sell a token that lasts until the order is completed, expired, or cancelled.
Any order older then 28 days may be automatically canceled by the system and all reserved funds will be returned to your account. If you have orders that are older than 28 days and the system has not canceled them, it is best to cancel and resubmit these orders to avoid cancellation.
An Immediate-Or-Cancel (IOC) order is an order to buy or sell a token that must be executed immediately. Any portion of an IOC order that cannot be filled immediately will be cancelled.
To learn more about Bittrex trading rules please click here