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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