Available is the quantity of an item that you own minus the Qty committed to Orders. In other words, On Hand minus Allocated. However, in MRP you may change names.
Big Business records that a Sales Order is Placed and sets Qty Open for items ordered. Qty Open is added to Allocated, which is subtracted from On Hand to find Available.
Allocated is the Qty you owe for Placed or Partial orders and is cleared when invoiced. For MRP, you might change the Label for Allocated to a name like Demand for clarity.