You'll see payment reference numbers (abbreviated Payment Ref #) on the Client Account Details screen. A unique payment reference number is assigned to each item sold. So, while one sale ID identifies an entire transaction, it might be linked to multiple payment reference numbers.
The number is linked to the Manage Sales screen, where you can view specific information about the sale, including the sale ID. The Sale ID identifies the entire sale, and the payment reference number identifies each unique item in that sale.