An AON is a type of order which is only executed if it can be filled completely. There need to be enough shares available to fill the order entirely, otherwise, the order is canceled. AON is only available for orders of more than 100 shares, thus 100 or more shares must be available for buying or selling.
Options traders need this kind of tool so that all the legs of a spread chosen are filled equally or as desired so that the spread is executed as per strategy.