what's the role and purpose of Qty f avail chk in sub po?

Hi,

when I view the components of a subcontract PO, I find a component with 3000 requirement qty and 2270 qty for available check. Then I recheck the availability of this component, the data don't change even though I have enough stock. I want to know the meaning of 2270 qty for available check which recorded in table atp_resb. And how can I change the data of qty for available check.

Thanks.