As for your first question, I'm pretty sure you can see the list of all environment variables that the KM Engine uses with this action:
The result of that command will also add any global variables, unless you remove them by clicking on the down arrow to the left of the word "printenv" and specify "Include No Variables."
As for "why would you want to know this information", I can't say why you would want to know this, but there are some potentially useful variables in there like SHELL, USER, PATH, PWD, LOGNAME, etc.