YoYo Games, the developers of GameMaker Studio, have posted a new tech blog article explaining variable scoping in the context of the GameMaker Language. Below is an excerpt.
“Scoping is all about the visibility of variables, as GML or DnD code is executed the variable names that are available to be read or written to change – what is available now is said to be in scope. So when a create event is executed any variables that are referenced are from the scope of the instance that is being created at that time.”