One of the main functions of an MRP Program is to tell you what materials you need, when you need them and, based on Vendor Lead Times, when you should be placing your purchase orders to ensure the materials arrive when required.
In MiniMRP you create your Works Orders and allocate materials to those work orders. The 'Allocation' process doesn't actually take the materials from your inventory. It simply tells MiniMRP what your future plans are.
MiniMRP can then look at your current inventory, your Work Orders and any existing Purchase Orders to generate a report showing you exactly what materials you need and when you need them. This is the MiniMRP "Shortage Forecast Report"