Quoting from PRO (Internet Printing Protocol/1.0: Protocol Specification):
Some attributes are encoded in a special position. These attribute are:
 “printer-uri”: When the target is a printer and the transport is HTTP or HTTP (for TLS), the target printer-uri defined in each operation in the IPP model document SHALL be an operation attribute called “printer-uri” and it SHALL also be specified outside of the operation layer as the request-URI on the Request-Line at the HTTP level. This
 “job-uri”: When the target is a job and the transport is HTTP or HTTPS (for TLS), the target job-uri of each operation in the IPP model document SHALL be an operation attribute called “job-uri” and it SHALL also be specified outside of the operation layer as the request-URI on the Request-Line at the HTTP level.
 “version-number”: The attribute named “version-number” in the IPP model document SHALL become the “version-number” field in the operation layer request or response. It SHALL NOT appear as an operation attribute.
 “operation-id”: The attribute named “operation-id” in the IPP model document SHALL become the “operation-id” field in the operation layer request. It SHALL NOT appear as an operation attribute.
 “status-code”: The attribute named “status-code” in the IPP model document SHALL become the “status-code” field in the operation layer response. It SHALL NOT appear as an operation attribute.
 “request-id”: The attribute named “request-id” in the IPP model document SHALL become the “request-id” field in the operation layer request or response. It SHALL NOT appear as an operation attribute.