SPQ (Supplier Purchase Quantity) Calculation in AIP

Hi,

Is there a document which explains the SPQ (Supplier Purchase Quantity) calculation steps in AIP?

The problem we have is where a forecast receipt at the start of the existing receipt plan disappears when a whatif (constrained) calculation is done after setting the SPQ value.If an SPQ is capped would it still bring in the orders at the start of the week and remove the orders at the end of the week ? How is the cap value spread between different ordering opportunities with in the week? Is there a priority matrix for the destination stocking points to get the share of SPQ cap?