Some inventory will be considered "fast-moving" if the on-hand supply is exhausted very quickly.
In some cases, it may be best not to guarantee inventory availability beyond a certain date even though you currently have sufficient inventory available. If the item in question is a fast-moving item and you can expect to exhaust all on-hand inventory long before an order is to be shipped, you may not want to hold inventory for that future order.
A good example of this can be found in the recent launching of a popular toy. Retailers refused to guarantee availability two weeks from the launch date. They correctly assumed that they would exhaust all available and expected inventory long before orders were due to be shipped.