What is an all-or-none order and why would I use one?

An all-or-none (AON) order tells your broker to fill your entire order at once or not at all, preventing a partial fill. Normally a large order for a thinly traded stock might execute in pieces over time as buyers or sellers appear, which can leave you with fewer shares than you wanted or multiple partial executions. AON avoids that by requiring the whole quantity to be available. The trade-off is that your order may take longer to fill or never fill if there isn't enough volume at your price. AON matters mainly for larger orders in low-liquidity securities. For the typical investor buying a few shares of a heavily traded index ETF, it's unnecessary, since those orders fill instantly and completely anyway.