External Procedure Calls 

The additional loaded program is an instance with its own global data area. When you call an external subroutine, the calling program and the loaded program form a single program group, unless the loaded program is a function group. Function groups always form their own program group. Furthermore, when you call methods externally, they and their class form their own program group. Within a program group, interface work areas and the screens of the calling program are shared. Classes cannot contain interface work areas.

All procedures, that is, subroutines, function modules, and methods, can be called externally from any ABAP program.

• You are recommended not to use external calls for subroutines, in particular if the subroutines share interface work areas with the calling program and call their own screens. The program group into which the main program of a subroutine is loaded depends on the sequence of the calls. The sequence is often not statically defined, but changes depending on user actions or the contents of fields. For this reason, it may not be clear which interface work areas and screens will be used by an externally-called subroutine.
• Function modules are intended for external procedure calls.
• Accessing an externally-visible method of a global class counts as an external procedure call.

You can call a subscreen screen externally by specifying an ABAP program other than the current program in the CALL SUBSCREEN statement. This program is treated in the same way as in an external subroutine call. In other words, it is loaded into the program group of the calling program, or, if it is a function group, as its own program group in the same internal session as the calling ABAP program.

For this reason, you should not use external subscreens that do not belong to a function group, since the same problems can occur with interface work areas and screen calls as can occur with external subroutines.