As the name suggests, an AON order, once placed, must either be filled in its entirety or not at all. This prevents any partial filling of orders and gives the buyer or seller the confidence of the fixed execution price for the entire volume.
This type of orders can be used, for instance, in algorithmic trading as a way to gauge the directional trend of a market and to prevent partial fill situations, which may happen when a market presents high levels of volatility with low volumes, such as during a flash crash type event.