How does AP_APY2015 order vouchers for payment?

In other words, there are a set of vouchers ready to be paid, how does the program decide that:

In other words, there are a set of vouchers ready to be paid, how does the program decide that:

voucher #5 = payment #1

voucher #3 = payment #2

voucher #7 = payment #3

voucher #2 = payment #4

etc..

Based on the payments, it looks like the program puts them in alphabetical order but, I am only looking at a small set and I don’t know if that is really the rule. And I don't know where to look for this configuration.

Thank you!