There is a hose and value attached to the coolant surge reservoir. The hose does not attach to the surge tank and dosn't seem to be part of the cooling system. One end goes into the engine or Transmission (can't see where it attaches), the other end has what looks like a valve and a yellow cap. Picture attached.

What is it used for?

 

My guess would be a vent or how about a way to insert a dipstick to check trans fluid?