If service engagements are performed by external executors you will see their Supplier code. If you check the box Purchase order for service, a purchase order will be created and can later be printed in the Print Purchase Order procedure. Here you will also see Purchase order number, Supplier invoice number and Amount.
The settings "Service or part when purchasing service work" and "Service code/part number for purchase of service work", under Service Orders under the Sales tab in the Settings procedure, determines how this will work.