How to pull FreePBX variables?

Question

freepbx

Hello dear community. Please tell me how to study. I'm interested in pulling out the variables used by FreePBX to better understand its capabilities.

For example, in one article I came across the ${AMPUSER} variable. Great, but how to get a full listing (or something significant) with comments on what's what. Does it exist in nature, or do you need to climb into the intestines yourself and pull it out from there? Um.

I don't know, should be a simple question? )