Bottom line is get some experience with mIRC, then the help file will make more and more sense. Even if you have experience with other languages and tech manuals, you still need to understand the IRC protocol to understand the mIRC help file to the depth you are requesting.

Maybe you should start by reading about the IRC protocol. I never did, I just got it from experience. When all fails, /echo is your friend. You can use it to tell you exactly what an identifier returns in an event.

As a general rule, if #channel is not in the event you won't be able to use # or $chan to return a channel name.

on *:join:#:{ <== Channel is in event.

on *:quit:{ <== Channel is NOT in the event.