will look at docs on R-xxPOST.MAC.
The process is quite simple, the only supplier they should be allowed to use is that supplier that is listed in the supplier field in the stock file.
however, if you create a manual purchase order, you are able to select any product in the stock file. A manual selection of supplier code in the supplier of the finder will only list parts from that supplier, hence the idea of forcing the entry of supplier code into the supplier field of the finder.
The issue is not relevant for automated purchase order as this already restricts the part to supplier listed in the stock file.