A more thorough explanation:

Compulsory joinder, also known as mandatory joinder, is the process of joining parties or claims to a single lawsuit. It is required for the lawsuit to proceed. Failure to join a compulsory party may result in dismissal of the lawsuit.

In civil procedure, Rule 19 of the Federal Rules of Civil Procedure governs the compulsory joinder of parties. This rule is intended to protect a party's right to be heard and to participate in the adjudication of a claimed interest. Rule 19 states that a person who is subject to service of process and whose joinder will not deprive the court of subject-matter jurisdiction must be joined as a party if:

• In that person's absence, the court cannot accord complete relief among existing parties; or
• That person claims an interest relating to the subject of the action, and is so situated that disposing of the action in the person's absence may:
• As a practical matter, impair or impede the person's ability to protect the interest; or
• Leave an existing party subject to a substantial risk of incurring double, multiple, or otherwise inconsistent obligations because of the interest.

For example, if a person owns a piece of property that is being disputed in a lawsuit, that person must be joined as a party to the lawsuit. If they are not joined, the court cannot accord complete relief among the existing parties, and the lawsuit may be dismissed.

In criminal procedure, the compulsory joinder rule requires a prosecutor to bring all known charges against a defendant arising from a single criminal episode in a single action. This rule is meant to protect defendants from government harassment and being subjected to successive trials for offenses stemming from the same criminal episode. Additionally, the rule promotes judicial economy by avoiding repetitive litigation.

For example, if a defendant is charged with assault and battery for an incident that occurred on a specific date, the prosecutor must bring all charges related to that incident in a single trial. They cannot bring separate trials for each charge.

Compulsory joinder is an important aspect of both civil and criminal procedures. It ensures that all parties with an interest in a lawsuit are joined to the action, and that the lawsuit can proceed without the risk of inconsistent obligations or repetitive litigation.