BIDI NOTIFY PORT SELECTED(21h)
BIDI_NOTIFY_PORT_SELECTED(21h) notifies the port driver that the port is selected in a print object. The spooler sends this command to notify the port driver that the output port is selected by a print queue.
BIDI_NOTIFY_PORT_SELECTED(21h) allows the port driver to know that the spooler has a print queue connected to this printer and that jobs may be submitted to this printer. This command can assist the port driver in determining the type of connection necessary for the printer.