Thanks - I did come to realize that the gauge was for DEF. As I understand it, once the tank goes to about 1/8th, it will start flashing yellow on the gauge - at this point, you'd better replenish the DEF fluid; because, if it allowed to go to flashing red, it will retard the engine.
FYI - My gauge just went down to the solid green 1/4 mark. I have about 3,200 miles now on my rig. I carry a couple extra gallons of DEF on board as well. I've just checked our local Pilot truckstop and they actually do have a pump that dispenses the DEF, while the local Loves and Flying J truckstop did not.