At invoice entry time, the sales rep as seen on the Customer Listing screen is filled in on the invoice by default. The data entry user entering the invoice has the ability to override the sales rep assignment to credit the individual sale and commission entry to a different sales rep. This is typically seen when more than one sales rep calls on the same customer.
Over the course of time, sales rep assignment to a customer will change. As they do, the new sales rep is assigned in the Customer Listing screen. When new invoices come in for that customer, the new sales rep is assigned on the invoice.