In addition to satisfying Rule 23(a)'s prerequisites, parties seeking class certification must show that the action is maintainable under Rule 23(b)(1), (2), or (3). Rule 23(b)(1) covers cases in which separate actions by or against individual class members would risk establishing 'incompatible standards of conduct for the party opposing the class,' Fed. Rule Civ. Proc. 23(b)(1)(A), or would 'as a practical matter be dispositive of the interests' of nonparty class members 'or substantially impair or impede their ability to protect their interests,' Rule 23(b)(1)(B). Rule 23(b)(1)(A) 'takes in cases where the party is obliged by law to treat the members of the class alike (a utility acting toward customers; a government imposing a tax), or where the party must treat all alike as a matter of practical necessity (a riparian owner using water as against downriver owners).' Kaplan, Continuing Work 388 (footnotes omitted). Rule 23(b)(1)(B) includes, for example, 'limited fund' cases, instances in which numerous persons make claims against a fund insufficient to satisfy all claims. See Advisory Committee's Notes on Fed. Rule Civ. Proc. 23, 28 U. S. C. App., pp. 696-697 (hereinafter Adv. Comm. Notes).
It is not enough that the threshold requirements of Rule 23(a) ...