External Subroutine vs. Internal Subroutine
Internal subroutines in the sense. subroutines which are defined and used in a same program.
external in the sense if you create a sub routine in one program and you're calling this subroutine in another program ..then this is external subroutine.
OR
The name itself implies the internal subroutines defined by form /perform.. can be called within the same prog in which they were declared.....external subroutines can be called outside the program.