If you model a support, say a "Y" (vertical) support for example, and you don't specify a CNODE and you leave the stiffness field blank, then you have defined a rigid vertical support. The associated node is fixed in space.
If you specify a stiffness on this "Y" support, less than the rigid default, then the node may experience displacement in the "Y" direction, depending on the pipe loads.
If you specify a CNODE, then the restrained node is fixed to the CNODE in the "Y" direction. Neither is fixed in space so both can move as dictated by the piping loads.
_________________________
Regards,
Richard Ay - Consultant