There is a difference between these two fields and is hugely important and not apparent. Found using the "Customer Transaction History" report and adding fields.
The "Sales Rep ID" is the Invoice Fact. The "Sales Representative ID" is the lookup to the Customer Record. The Customer Record does not have a field named "Sales Representative ID" which is already an inconsistency. Any data point should ALWAYS use consistent naming to avoid confusion.
Using the "Sales Representative ID" in the "Customer Transaction History" report may lead to inaccurate Commissions to Sales-people. While a Sales Representative may be assigned to a customer account, not all sales may be performed by that individual (Vacation, short-term disability, etc.) However, with the lack of clarity between these two fields, it would be very easy for any user to get the incorrect information.
Please update the names for clarity in reporting and consistency when trying to identify where the data comes from and can be changed/updated.