Salesperson vs. Service Rep

The Salesperson for a customer will be assigned by default to new quotes, orders, and invoices unless a user is linked to a salesperson. In that case the quotes, orders, and invoices entered by that user will be linked to their salesperson account.

The Service Rep for a customer is an informational field and is not assigned specifically to new orders and invoices created for the customer. The service representative is a person who may be a follow-up contact for the customer after sales have been made.

Report filtering for quotes, orders, and invoices in general is done by salesperson then and not service reps since service reps aren't tied to specific orders or invoices, however you can run the Income -> Reports -> Summary of Invoices report by service rep, and this will look at the service reps tied to customers.