I forgot to give an analogy that I was thinking about. 

Cookie jars are for storing cookies, no? What if I want to put coins in there - will they come back out as cookies? Will the cookie jar refuse to hold the coins since they are not cookies? Nonsense.

And if I place cookies in my coin purse, does the purse refuse?

Similarly, the red channel can store various amounts of red stuff, or it can hold bump map heights, if I choose to use it that way. Amount of red, amount of bump, both are numbers and can be stored in 2-dimensional arrays of numbers called images. I can store them any way I like, and different ways offer different advantages with respect to space, time, and convenience. (Never all three are good at once)

However, this is a Poser trick. Many other 3D apps could not deal with this because they do not have such good facilities for doing math with images. That's why you don't see this kind of magic used very much.

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)