What does EXT button do?

[photopilot]

My manual has a nice picture of the heater/AC control, but the words go with the older analog control. What does the EXT button do? The AC seemed intermittent for the first two days after we picked up the new to us coach, and I'm not sure if the EXT button had anything to do with it. Once I got the AC working, I haven't dared touch the EXT button since.

[Playtime III]

temp outside. The digital is not accurate on mine. I have to set it high for heat (85) and low (65) for the a/c to blow warm(cool) air

[windsorbill]

that's 'exterior temperature'. Mine runs about 6 degrees low. Somebody somewhere told me you can't calibrate them.