You can assign reason codes to transactions by using reason codes on sales and purchase invoices.
Go to the Sales Invoice window or the Purchase Invoice window.
On the sales or purchase header, enter the code in the Reason Code field. (This field is not included in the standard layout, but you can insert it with the Design facilities.)
When the invoice is posted, the reason code is copied to each G/L, 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.
Assigning Reason Codes to Journal Batches
Assigning Reason Codes to Journal Templates
Assigning Reason Codes to Journal Lines