It seems like a global variable can be passed another script within a given database (e.g. if the parent script calls a subscript with the "Perform Script" step) but not if the subscript is in a related Filemaker database. Is that true?

I suppose I'm stuck using script parameters.

The scope of $$vars are restricted to the current file.

The scope of $vars are restricted to the current script.

So, yes, you must use script parameters to pass from one file to another, but there is another way...

Using a relationship between two files you can reference global fields (not $$vars) in the other file.

That sums things up nicely, thanks.