If you leave RX blank, you imply that the node provides no restraint against rotation about the X axis - it is free to rotate; no stiffness.

If you set RX to zero, you imply that, no matter how much bending is supplied by the piping system, that node will not be able to rotate about the X axis - it is fixed in rotation; it had infinite stiffness (in CAESAR II that's about 1E12 lbf/in).

We're talking about imposed displacement but the focus is on stiffness - the stiffness of the nozzle/shell connection, not the flange face or weld end of the nozzle. Of course that stiffness need not be only zero or infinite.