In the Sales and operation planning (SOP), 'Available quantity' is not taken into account on purchase orders (SPO's) that do not have a confirmed delivery date.

To consider this quantity too, there is a settings that can be activated:

Treat supplier backlog as stock for purchase requirements 

By enabling this all purchase orders and purchase order responses will be treated as stock. 

Any purchase orders and purchase order responses confirmed in the future (ex. in two months) is available from today and can be used to net off purchase requirements. 

By not enabling this, you might end up with purchasing products that could instead be acquired by requesting change of delivery date on existing supplier backlog.

Read more about the  Sales and operation planning (SOP) application here: SOP application