What are the types of Subroutines in ABAP? | ABAP ALV

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