PIR Reduction & PIR Consumption

Hi Team,

I am reading the below statement in SAP forum.

"In many make-to-order production strategy, sales orders or their deliveries consume the quantity of planned independent requirements. After issuing goods by delivery for sales orders, the quantity of planned independent requirements are reduced".

What is the difference between PIR consumption & PIR reduction? Can anyone help me to understand the difference between PIR reduction & PIR consumption with some example?

Thanks