(series implies the first major release of NX, i.e. NX1953, NX1980, NX2007, etc.)
Or, the appropriate Windows path in the inch folder if used by default.
NX is expecting customers to create their own PTB files, which will require their own custom bend table. So instead of NX having a value in the customer default OOTB (out of the box) which would be equally confusing, it was left empty.