This lists all invoices for a particular client, whether paid or not. When the list gets very long use the search tool at the top of the form to search by invoice number or date.
This list is accessed from Cheques/payments (for client refunds) and Client Invoicing. And from the POS forms when recording an invoice payment for a regular customer. The client's invoices displayed are those of the client displaying on the form the list was activated from.
When accessed from the Client's invoice form clicking the select button displays the details of that invoice in the form. If accessed from the Cheques form selecting an invoice links the cheque/payment to the selected invoice - providing you are editing or adding a record.