What must be true for a party to be considered indispensable under Rule 19?

For a party to be considered indispensable under Rule 19, it must have an interest in the subject of the action. This means that the party's absence from the litigation would impede their ability to protect that interest, or would leave existing parties vulnerable to multiple or inconsistent obligations. The necessity of having a party who possesses a clear stake in the outcome ensures that all relevant claims and defenses can be fully addressed.

In distinguishing this option from others, while aspects like residency (which refers to jurisdictional considerations) or being a co-defendant might be relevant in some contexts, they are not criteria for being deemed indispensable under Rule 19 specifically. Additionally, the availability of a party for testimony does not dictate their indispensability—instead, what matters is whether they have an essential role in relation to the case's subject matter.