Function group is loaded into runtime memory by the FIRST call of a function module inside this function group.
See example below:
I have a global variable defined in function group in X3C/504:
Before I call any of the function module in this function group, this global variable is not available - this make senses because the whole function group is NOT loaded into memory.
When the first function module is called, the function group is loaded into memory - global variable available:
I insert one entry to this table:
Now function1 execution is finished, when I entry function module2, this global variable is still available, because the lifetime scope of function group is application level - which means it will always stay in the memory until the application terminates.
概括成一句话：function module里定义的局部变量，作用域是function module scope，即module执行完变量就失效，但定义在function group级别的全局变量，生命周期是整个应用，即应用不关闭之前，一直有效。
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Global variable in ABAP function group
原文：https://www.cnblogs.com/sap-jerry/p/12056412.html