Failure simulation. This is what in my opinion takes the DC-9 Classic, and all other addons that includes such a feature, to the next level. Usually when you simulate a system in an aircraft, you simulate ON or OFF in a normal state. With failure simulation a system can also be simulated ON or OFF in a failed state. While OFF is usually OFF regardless of state, ON and failed at the same time is where it can get really busy real fast on the flight deck.
What exactly is specularity and how does it work in FSX? On the Coolsky support forum McPhat Studios explains in detail how specularity works on the new upcoming soon-to-be-released freeware American Airlines UHDT livery.
The DC-9 Classic comes with these 6 liveries in the release package: