Don't use Global variables for Apps Script Services. For example:
var SS_SERVICE = SpreadsheetApp;
Recently this started causing an error message. If this changes, please edit the answer at that time.
Also, if you do not use the var keyword to define a variable, then it automatically gets put into the global scope. So, if by mistake, you fail to put var in front of your variable, then the code still runs and may work, but you may be unaware of what is really happening with your code. If you defined and used another variable with the same name in a different function, and also mistakenly made that variable a global, and one function called the other function, then there could be a conflict with the variable values.
All Apps Script .gs files can access all other .gs script files. There doesn't need to be any link between script files, or inclusion into other script files. You can call a function from another script file, as long as it's in the same project.
And global variables defined in one file are accessible to other files.
You don't want to use public Cache for information specific to that user. But there is private Cache. And Cache expires, so unless it's for something like timing how long a user is logged in, you might not want to use it.
If you have lots of code, and create functions for reasons of orderly structure and access to multiple other functions, then passing data might be undesirable. So, yes, you can use global variables. It's considered "Bad Practice" by some to use global variables, but then we are getting into personal opinion.
