Which VI or function are you referring to? There is neither an "IVI New Session.vi", nor a corresponding "Ivi_NewSession" function in C.
The Ivi Class Drivers determine which specific driver to use at run-time, by looking up the resource descriptor (a Logical Name, or Virtual Instrument) and determining the associated Instrument Driver. That configuration information is settable by the user through MAX (Measurement and Automation Explorer).
--Bankim
Bankim Tejani
National Instruments