Mike,
The instrument handle is a variable that is created each time the switch is run. This is a reference to all the configurations that were setup for the switch and it is destroyed when you close the session. If you take a look at the examples that ship with NI-SWITCH you will see that the the first VI, NI Switch Initialize, takes in a instrument descriptor, which gives some information on the configuration of the switch. The output of this VI is the instrument handle that is used by all subsequent switch VIs. This handles will always need to be closed out or you will not be able to access the switch again until you reboot. You will not be able to use a constant because the handle is created each time the VI is run. The handle can only be created by the NI
Switch Initialize.vi.