What activates the 3 lights in Parking/Driving light assembly

Greetings everyone. Need some help with my parking light. I am replacing all of my light assemblies for my 2000 GMC Yukon SLT 4.8L. When I removed the parking/driving light assembly below my driver's side headlight, I noticed that are 3 lights in it from left to right and only the middle amber light worked. I replaced the two outer lights and found out the outside light activates with the left hand signal. I've still no idea what the inner light (closest to middle of vehicle) does. Tried putting car in park, forward, reverse. As it turns out, the same two bulbs are burnt on the passenger side parking light so no way I gain any knowledge from that.

Can anyone tell me when the inner parking light bulb activates on my car?

Will D