What is meaning of 'Finished Good Item Qty'

What is meaning of ‘Finished Good Item Qty’.

This shows up when creating a Purchase Order, specific to a subcontracting item.

While Creating a PO but before creating the Subcontracting order.

Click on the Item row edit button and it gives additional options.

Here the field mentioned shows up. See image.

What does this field signify exactly.
Since there is a Qty field already mentioned, what will this field mean in this context.