Payment budget and commitment budget

I need to know the difference between payment budget and commitment budget
is the commitment budget part of the total amount allocated on the payment budget or is it an addition to it

for example
if the total budgeted amount for a certain expence is 100$
and 50% of the purchases are through POs
does this mean the payment budget is 100$ and the commitment budget is 50$
or payment budget is 50$ and commitment budget is 50$