Refnum is No Longer Valid

Hi Guys:

     i met a "Refnum is No Longer Valid" issue in following case:

1 In B.vi , "Open vi reference" of A.vi(option x100) , Start Asynchronous Call , and store the reference into a global variable

2 In C.vi,  use "Wait On Asynchronous Call" to queue the status of the A.vi instance calling from B

     Problem here is , when i use the global variable in C.vi , it shows the correct memory address but "Refnum is No Longer Valid" , i know there's already some discussion about this , saying that all the reference will be clean after vi finish , solution is like "The simple (and correct) solution is not to call the functional global by itself, but only to call it as a subVI." , i don't quite understand what does this mean , any suggestion ?