can someone tell me what they do? and how are these things used?

is there like a warning light on your console when your rear bumper's within a certain distance from the wall/another car/ etc?

2nd question: can this be installed as an option? aftermarket parts?

thanks!

 

Not sure about nisssan but the one that I saw in a ford beeped more rapidly the closer you got to something while backing up. Which I had one today when I bumped the curb with the front.