Delivery Priority effects on ATP Check

Dear All,

I have a two customers, 1. delivery priority is set as high and for second customer delivery priority is set medium.

now in SAP user create the sales orders separately for both customers of a same material.

How delivery priority can be effect while doing the ATP check, is there any customization required, i want system first reserve the quantity for a high delivery priority customers even the SO later after medium priority customers.

Can any one explain how it will work, thanks.