It's not a method, it's an assignment. However, 'dim' in most languages (this looks like VB) don't return anything, so the construction is like wearing shoes on your head.
Yeah, DIM is usually a declaratory, so the statement seems to be saying if it's defined undefine it... Though really it doesn't quite look like VB either... Looks more like gibberish. But then the 'sentence' after the code is incomprehensible too so...
Don't take this the wrong way, but ENGLISH, Do you speak it?!? If not, we might not be able to help you as you're still posting incomprehensible gibberish that fails to even explain what you are trying to do.
Yes, and it doesn't return anything, so if(Dim ...) doesn't mean anything. No method. Variables are assigned values as an internal function of the compiler. What you're asking is the equivalent of what store do you go to in order to digest your food? You don't - digestion is an internal part of you.