For many kinds of inventory items, there is flexibility in the quantity of goods delivered within a reasonable threshold. For example, contracts for wiring often have overage and underage clauses where the vendor can deliver 10% more or less than the contracted amount. In the case of wire, this is because wire comes on a spool, and it is inefficient to leave a small amount on a spool or start a new spool for an order that an existing spool can't quite fulfill.
Without a mechanism to deal with this, Purchase Order's for these items have to be closed manually for short orders or adjusted upward for overages. Dynamics GP 2016 has features in place to handle tolerances such as this.