Indicates the state of system main power. ("on" will be sent when the unit is powered on, and "off" when it is shut down.) No messages are sent when the unit enters or leaves auxiliary power mode. For various reasons, such as in response to the power-q message, this may be re-sent without the unit changing power state, so any devices that have boot-up or shut-down actions should store the current power state and verify that the power state has changed before taking action.