It's a kind of security offering. Either the whole thing gets done - or nothin' gets done. It applies to IPOs and stock block trades. All or none block trades are issued this way by the buyer in cases where they are either famous or known for trying to buy out a company entirely. Imagine if Warren Buffett bought 2.3% of Netflix (yes, it would mean the world is ending but that's a separate story). NFLX stock would almost certainly shoot up a ton. So if Warren wanted to own 10% of the company, he'd go out to brokers for an all or none order with that 10% "all" figure. Once he has already bought his stock in the amount he wants to own, if NFLX then shoots up, fine. No sweat. Thanks for the easy money. But if he owns 2.3% then he's in a kind of "no man's land" and either has to pay up a big price for the remaining 7.7% he wanted to own. Or turn around and sell the 2.3%.