In the case of a joint and several obligation on the passive side established by an arbitral award, the challenge of the award does not require the necessary participation of all the joint and several co-debtors, given the permissive joinder that characterises the procedural relationship between joint and several co-debtors; it follows that the failure to join a joint and several co-debtor does not give rise to a requirement of necessary joinder pursuant to art. 331 cod. proc. civ., but rather to the application of art. 332 cod. proc. civ. concerning severable causes of action.
The inseverability of causes of action in proceedings for the challenge of the arbitral award, with consequent necessary procedural joinder pursuant to art. 331 cod. proc. civ., arises only where a dispute arises between the parties as to the identity of the debtor or where the decision between claimant and defendant is in the alternative to that between claimant and third party, so that the various procedural relationships become inseverable by reason of the interdependence of the causes of action.