PARTIES (IN JUDICIAL PROCEEDINGS). — They are the natural or juridical persons who either seek or against whom the application of the law is sought, and who act in their own name for the protection of their substantive right. Title IV of Book IV of the CIC (De partibus in causa) illustrates in practical form the concepts elaborated by doctrine. By the term "parties" one commonly understands the passive subjects (in contrast to the judge, the active subject) of the process, namely those who defend interests or rights within the process. But this is not yet sufficient to clarify the concept of "party." According to can. 1648, for example, the representative acts in the process, but the party is instead the represented person.
The quality of party is determined by the judicial petition. From can. 1648 it follows that in every process there must necessarily be two parties: the one who seeks and the one against whom the petition is made, i.e., the plaintiff and the defendant. The person who acts, that is, who introduces the cause (in canon law by means of a libellus [v.], in Italian law by citation), must also indicate the substantive right on which the petition is based and the violation or imminent danger of violation of that right. These elements, contained in the petition, confer upon the plaintiff the quality of party and confer upon the defendant the quality of resisting party. It should be noted, as mentioned above, that one who acts in the name of another is not a party in the process; rather, the party is the person represented, who will derive any advantages or disadvantages from the judge’s decision. A typical example is the organs of juridical persons, who have the capacity to appear in court but not the title to the substantive right in dispute, which belongs instead to the juridical person, who also has the status of party. For the purposes of procedural law, the determination of party in judicial proceedings is of the utmost importance; as a first consequence, the rights and obligations of procedural law and participation in the juridical relationship of the process flow from II.
The status of party is moreover absolutely necessary in order to establish active or passive standing to sue or to be sued, the competent forum ratione domicilii or contractus, the right to free legal assistance, and so forth. To the party certain activities are forbidden, while others are exclusively reserved to them. The party, for example, cannot be a witness or expert in its own cause, cannot perform any indirect or mediated procedural activity, such as intervening as a third party or proposing a third-party opposition to the judgment. To the party, on the other hand, belong all those activities that lead to the tribunal’s decision and the lodging of appeals against the judgment. In a single process multiple actions may be joined and multiple parties may be involved, but the positions are essentially two: plaintiff and defendant, even when the parties assume other titles (appellant and appellee, opponent and opposed, creditor and debtor). In canon proceedings, which have as their object the declaration of nullity of the sacred bond, besides the spouses the Defender of the Bond is also a party to the process, who must oppose the petition and thus assumes the role of defendant. In penal proceedings, whether canon or civil, the one who acts is the public prosecutor (promoter of justice, public prosecutor), and the defendant is the accused.
The parties in the process stand in a formally equal position, enjoying identical rights and obligations, but their substantive positions differ: the plaintiff is not compelled to act (except for the promoter of justice in cases prescribed by law—cann. 1955, 1971—or others bound to act by particular provisions of law), but if he acts he must prove the fact alleged in the petition, the right violated by that fact, and his title to that right; the defendant, on the other hand, is always bound to respond, and his position is more favorable because he is brought before the judge without his own concurrence and need not prove anything, but only raise exceptions. The famous maxim "actore non probante, reus absolvitur" clearly indicates this. The CIC, in the aforementioned title, regulates the capacity to appear in court, granting this capacity to some and denying it to others. In the second chapter of Title IV, the CIC sets forth the norms that must be followed by persons who act in the process in the name of another ad adiuvandum, namely, procurators and advocates.