The Inventory Available to Promise screen will give you insight into your current inventory situation for any item. For any item that you select in this screen you will see the supply and demand for the item in a time sorted list. You can drill back into any listed item to get more information.
The definition of Free-forward quantity is the total of available and pending quantities of an item, plus any additional quantities from purchase orders, assembly receipts, manufacturing orders, and sales returns displayed in the scrolling window. The total sum is reduced by the item quantities from unallocated sales orders, sales back orders, and unallocated manufacturing components displayed in the scrolling window.
There is a feature that allows you to forecast when a new specific quantity of the item will be available, and when you could start a manufacturing order to build the quantity, if needed. Here is a good example that shows the various attributes of the calculation, and displays the related transactions:
In this example I wanted to know when a quantity of 200 would be available. I entered 200 in the Required Quantity field, and the results show that there will be at least 200 available on 6/15/2017. By quickly reviewing the list of transaction you can see that the manufacturing order for a quantity of 8o is the limiting factor for this result. If you click on the Calc. Mfg. Date button, a report will run that shows you the items required for the manufacturing order. In this case the manufacturing order is waiting for a supply of Black Ink that will be delivered on 6/12/2017.
The Inventory Available to Promise screen is also accessible directly from the Sales Order Transaction Entry screen by clicking on the blue expansion arrow just to the right of the QTY Ordered field label.
Here is a soon-to-be ancient brochure from Microsoft. GP-Available-To-Promise