After serious thinking williamwright wrote :
I don't know but mine has the same thing except it sounds a buzzer.

My fancy system indicates nothing at all if everything is working
properly - I'm not sure it is entirely legal to the letter of the law.

Rather it monitors all of the trailer lighting constantly and puts an
icon up on the screen, indicating which bulb has failed. Once on, it
stays on screen, until the engine is restarted - so no chance of it
missing an intermitant problem.