What is Proper Parties?

Legal Definition
A proper party, as distinguished from a necessary party, is one who has an interest in the subject-matter of the litigation, which may be conveniently settled therein; one without whom a substantial decree may be made, but not a decree which shall completely settle all the questions which may be involved in the controversy and conclude the rights of all the persons who have any interest in the subject of the litigation. See Kelley v. Boettcher, 85 Fed. 55, 29 C. C. A. 14; Tatum v. Roberts, 59 Minn. 52, 60 N. W. 848.
-- Black's Law Dictionary