Optional connecting node. Restraints with connecting nodes can be used to tie one node in the piping system to any other node in the system.
If the connecting node is specified then the restraint node is tied, via the restraint stiffness, to the connecting node. Also if you want to have member forces been shown in restraint report, you can set neighboring nodes tied by Cnode in case of flange faces.
On the other hand, Cnode is used to give displacements at restraint via the node not used in the piping model. It is because CAESAR II does not permit to have restraint nodes move. The typical examples are restraints at nozzles where thermal movements are expected.