This article is from the The Monster Truck FAQ, by Jeff Hawkins with numerous contributions by others.
A. There are two or three lights mounted in the windshield of most
monster Trucks. One tells the track crew and starters when the
engine is actually running. The other two lights tell the track
official(s) who run the Remote Ignition Interrupters which side
the driver has chosen to run his `RII' receiver box on.
You will see a 2" X 2" sticker in the windshield with a number on
it from 1 to 10. This number is the radio channel number. A letter
also may indicate right or left for use during single performances.