Certain payment types can be configured to capture a voucher code (via a modal window) just as additional information (stored in JPDETAILS1-4 field - table s_jhead).
In order for this to work, there must be a voucher='Y' setting in the general supervisor fields (c_genera).
For each payment type that should capture the voucher code there needs to be an entry in menu Settings, Combox Lists:
Field: 'vouch_info'
Code: 'ptype1', 'ptype2', ...
Control: The payment type, e.g. "VOUCHER".
The codes 'ptype1', 'ptype2', ... do not correspond to the four payment type rows on the billing form. They are numbered to allow for an unlimited number of voucher info payment types. If you only need one, just use 'ptype1' (although 'ptype2' will work as well).