Abbreviations on school bus / truck fan switches

Factual Questions
1

This is going to be a really useless thread, but I just want to satisfy my curiosity. As a kid on the school bus, I would look at the driver’s command panel. I noticed that there were switches marked “FAN AUX DEF” and “FAN RR HTR”. I tried to decipher these abbreviations. What exactly do they mean?

Is the first one for some kind of auxiliary defogging device? And if so, what is is it auxiliary to?

I assume the second one is for a rear heater? In this catalog of school bus parts, there is also (p. 260) besides the switches already mentioned, a “fan aux[iliary?] h[eate?]r” and a ''fan r[igh?]t h[eate?]r" .

2

Auxiliary defrost (or defog) and rear heater. The defrost would be a fan mounted on a stand aimed at the windshield. The windshield is too large for the normal hot air nozzle to reach all of it.

3

Cool. So far, I think all the questions I’ve asked on these forums have received a satisfactory answer - no matter how nitty-gritty.