Just as a block or function is nested inside another block or function, scopes are nested inside other scopes. So, if a variable cannot be found in the immediate scope, Engine consults the next outer containing scope, continuing until found or until the outermost (aka, global) scope has been reached.

Consider:

The RHS reference for cannot be resolved inside the function foo, but it can be resolved in the Scope surrounding it (in this case, the global).

The simple rules for traversing nested Scope: Engine starts at the currently executing Scope, looks for the variable there, then if not found, keeps going up one level, and so on. If the outermost global scope is reached, the search stops, whether it finds the variable or not.

To visualize the process of nested Scope resolution, I want you to think of this tall building.

You resolve LHS and RHS references by looking on your current floor, and if you don’t find it, taking the elevator to the next floor, looking there, then the next, and so on. Once you get to the top floor (the global Scope), you either find what you’re looking for, or you don’t. But you have to stop regardless.