How to: Use Reason Codes on Sales Documents and Purchase Documents
You can assign reason codes to transactions by using reason codes on sales and purchase documents.
To use reason codes on sales and purchase documents
Open the relevant sales or purchase document.
On the sales or purchase header, enter the code in the Reason Code field.
When the invoice is posted, the reason code is copied to each general ledger, customer, and vendor entry. You cannot assign different reason codes to the individual purchase and sales lines because all lines are posted as one entry.