Heater box door explanation?

RustyRatRod said
In your second picture, Vixen, my 64 Valiant does not have the door second from the right. She only has three doors, as she only has three control knobs.

Ok and so that’s what I was looking at, that second door from the right blocks off the front or rear of the heater core based on the position of the door. Not sure that actually does anything? Thinking of just removing that door and running the most outward rt door cause it would block off heat for fresh air.