Program Calls - Overview
In a program call, the unit that you call is an independent ABAP program. At runtime, the called program is loaded into its own internal session in the current external session (memory area for a user session). Any program that can have its own screens can be called in an external program call. The most usual external program calls are for executable programs and transactions assigned to a module pool. At every external program call, a new internal session is opened, and therefore every call triggers the event LOAD-OF-PROGRAM.
When you call an executable program, the program is loaded, and the ABAP runtime environment calls the processors that control its flow.
When you call a transaction, the program linked to the transaction code is loaded and its initial screen is processed. The initial screen calls dialog modules in the called program, and then branches to its next screen.