2006 Ford Escape Hybrid tail lights

OK. Here's a dumb question.
Yesterday a guy pulls up beside me and says that my left brake light is out.
When I get home I checked the tail lights and the brake lights were working
fine. But I noticed that the other red light, second from the bottom, seemed to
have no function. There are four lights on the tail light assembly. Two red, one
yellow and one white. The running light uses the same top red light as the
brake. The white is for reverse and the yellow is for signalling. But I couldn't
get that bottom red light to come on on either side. What is it for? I looked in
the owners manual, but it doesn't describe the tail light assembly.