Figure B-11 shows a Flight Strip Printer that is modelled as a serial item; however, it is not required for system operation. The block diagram indicates that a failure of the Flight Strip Printer will not prevent the system from completing its mission. The effective failure rate of this sub-system is therefore zero and is normally included in the total failure rate for the system.

A sub-system not required for system operation is often shown with a shaded outline and an explanatory note as displayed in Figure B-1.