$$ is used as a way to stop output if there isn't a value. It's useful when you don't want to throw an error message if some value isn't included. Of course, you can include error checking to verify that the value is there, but $$ is easier and does the same thing. A basic example is something like:
msg $chan $$?="Message"
Without the $$, it will give an error if you press cancel or ok without any text. With $$, it will simply halt the msg without throwing an error.
As far as naming identifiers, yes, you can name an identifier something like $identifier if you want to. However, it will be of limited use. You cannot call it using $identifier or $$identifier because both will point to an identifier named "identifier". $$identifier will just halt if nothing is returned. You *can* call it using $$$identifier. The limitation is that this will always halt if nothing is returned. If that's okay with whatever your script is doing, then it isn't a problem. But, if there are times with the identifier can return $null and you want the script to continue without halting, then that's not going to be possible with that naming scheme.
