A more thorough explanation:

Mandatory joinder is when a person who is not an original party to a lawsuit is required to be included in the lawsuit because their presence is necessary for the case to proceed. This person is called an indispensable party.

• In a divorce case, the couple's children may be considered indispensable parties because their interests would be affected by the outcome of the case.
• In a personal injury case, the defendant's insurance company may be considered an indispensable party because they have an interest in the outcome of the case and would be responsible for paying any damages awarded.

These examples illustrate how a person's presence in a case may be necessary for the court to provide complete relief to the existing parties or to protect the interests of a person who would be affected by the outcome of the case.

In federal civil cases, the Federal Rules of Civil Procedure 19 govern mandatory joinder. The rule states that a person is only indispensable if their inclusion in a case would not deprive the court of subject matter jurisdiction. If a court cannot join a party because it would ruin the court’s jurisdiction or is infeasible for other reasons, the court must decide whether it can continue in “equity and good conscious” without that person or dismiss the case.

In criminal procedure, mandatory joinder refers to the joining of all known claims against a defendant in a single prosecution. Some jurisdictions also require that the claim arises from the same criminal transaction. Any known offenses not joined as a separate count within the same case cannot be the basis of a subsequent prosecution.

Mandatory joinder is an important aspect of the legal system because it ensures that all parties who have an interest in a case are included and that justice is served.