When does the Purchase order Header Order Amount reduces the Amount

1. initially the PO header total cost is the total extended amount of the Order detail lines

2(a). when the PO line is cancelled - does the Order header total amount is reduced by the cancelled ampunt.

                                              Or

2(b). Is the PO header total amount equals to amount received.

I think it is 2(a), please let me know if this is correct.

thank you