3.2.5.3.2 Sending a Register Request Callback Message
The structure and fields of the REGISTER_REQUEST_CALLBACK message are specified in section 2.2.6.2.
The server sends the REGISTER_REQUEST_CALLBACK message with the RequestCompletion field present to the client in order to provide a unique Request Completion Interface for the client to use. The server MUST send this message once for the same USB device and it MUST send this message before sending an IO_CONTROL, INTERNAL_IO_CONTROL, TRANSFER_IN_REQUEST, or TRANSFER_OUT_REQUEST message.
The server sends REGISTERS_REQUEST_CALLBACK message without the RequestCompletion field in order to stop the client from sending any messages on the Request Completion Interface (section 2.2.7).