Do the variables that are used in an external subroutine (like #12 = 42) affect the variables that are used in the main program with the same name (like #12 = 37.5)?
I am aware that variables in the main program are public to the entire main/local module.
Will the variable #12, if used, in both the local program and the external subroutine interfere with each other? If they do, that means each subroutine must be uniquely constructed.
I am aware that things like G52 are not buffered and if incorrectly used, will return to the main program to an incorrect pointer.

Another item that I haven't completely explored yet. Will the local subroutine numbers (like O10, O11, etc) in the external program module interfere with the main program subroutine numbers of the same number? My G-Code generator reuses local subroutine numbers on each project build, and it is highly likely that creating a general purpose subroutine will have subroutine numbers that are identical to the numbers in the main program.

I could find no documentation that lays these rules out.
What are the rules here? 
Thanks,
Bill

Everything in Mach is global. This is different from Fanuc.
So the opportunities for confusion and chaos are almost unlimited.

Cheers
Roger