Intermarket sweep order
Intermarket sweep orders (ISO) sweep several different market centers and scoop up as many shares as possible from them all. These work against the order-protection rule. For example, if a trader is trying to buy 100 shares of X, and there are 10 shares of X being offered at $1 at one exchange and 1000 at $1.10 at another exchange, the order protection rule would let you buy ONLY those 10 shares at $1, after which you would need to send in other orders. With the ISO, you can buy the 10 shares at $1 and the remaining 90 at $1.10 on the other exchange subsequently.
References
BATS Exchange Order Types and Definitions
WSJ: Accenture 'flash crash' and Intermarket Sweep Orders
Minyanville: How ISO's May Be Used as Predatory Trading Tactics
Interactive Brokers (Intermarket Sweep)