Field help (REPSIH) 

The sales representative codes are modifiable. They are initialized, in the case of a direct invoice or credit memo, by the sales representatives of the bill-to customer and if there are no representatives in the bill-to customer, they are initialized by the sales representatives associated with the delivery address.
If the invoice is associated with an order, or a delivery, etc., the sales representatives will be initialized by the sales representatives from the first original document.
In the case of a prospect, only the prospect's sales representatives will be considered. These sales rep codes will be copied to the document lines. If a representative is modified in the document header when this is created or if lines exist in the document, the system will propose to copy the modification to all the lines that already exist.

From these fields, the user can directly enter a sales representatives whose existence is controlled in the corresponding table or use the contextual button to: