Change Need by Date in PO line, CPA was assigned in Source Document field

in 11i, when we modify the need by date in PO line, the item is related to BPA, BPA number will be assigned to PO line source document.if no BPA created, item will not linked with CPA.

But I found R12 will source the CPA number into PO line when need by date updated by user. May i know is it standard behavior in R12? how can we fix it?

And i don't know why CPA can be marked on Source document field, in 11i, CPA can be marked on Contract field in PO line.

Please kindly advise. Thanks.