Internal Subroutines: The source code of the internal subroutines will be in the same ABAP/4 program as the calling procedure (internal call).
External Subroutines: The source code of the external subroutines will be in an ABAP/4 program other than the calling procedure.
External Subroutines: The source code of the external subroutines will be in an ABAP/4 program other than the calling procedure.