a Variable is a Variable only after you have announce on him!!!!!
After your announce set variavle :var_name = var_value)
you can do what ever you want with it.
Now, if you have announce on a Variable in frame 3 , you will not be able to use it in frames 1 or 2!!!
But, after u have announce on it , you can call it as many times you want, from any place you want (even from out of of flash!!!)
At last, if u want to "kill" a Variable, simply give him a new value, or give him a null value.

Hope it helps
ND