I have found this url
http://www.chsoft.com/mmfaq.html
Here's a sample from my document:
Enter a negative non PO voucher corresponding to the global amount of retrieved vouchers
Release vouchers blocked for payment if necessary
Calculate the global amount of retrieved vouchers
Retrieve all unpaid vouchers from the bad debt vendor
Is the vendor willing to pay his debts
Contact the vendor
(PO=Purchase Order)
These are steps of a procedure. I don't know whether I should associate positive with credit and negative with debit when it comes to vouchers.