Communication Between R/2 and an Externally Registered Program 
Communication Between R/2 and an Externally Registered Program
Purpose
Communication between the R/2 System and external programs is also supported by the SAP program interfaces.
The following topics provide an overview about the prerequisites for the R/2 host and the partner computer.
These prerequisites depend on whether R/2 is the calling system or the called system:
From R/2 to an External Program (registered program) From an External Program to R/2
As of R/2 Release 5.0D, you can also use an RFC call instead of a CPI-C call.
The following limitation applies to an MVS/VSE host: