VA41 contract computation

Hi experts,

in va41 we can create contract document. In VA01 we can create document with reference to the contract. After choosing contract we can select line that should be copy to new VA01 document. I want to know how can I compute available lines in contract document.

After creation va01 with reference do contract the available materials quantity is decreasing. Some lines in contrac can be used. Which tables should I used or mabey BAPI to compute available lines in contract ? I found VBFA but it seams not 100% correct.

Thanks,