When developers use custom code and gauges for their more complex aircraft, they often do not use Standard Datarefs and Commands and this means you cannot use normal X-Plane software, plugins and utilities to map the custom code and gauges to your hardware.
However an aircraft developer will often use hidden Datarefs and Commands to make the aircraft work. These hidden Datarefs are known as Custom Datarefs and the hidden Commands are known as Custom Commands.