In 1990, the National Labor Relations Board (defendant) promulgated the first substantive rule defining appropriate collective-bargaining units in a particular industry. The rule provided that eight, and only eight, bargaining units were appropriate in any acute-care hospital. The rule recognized three exceptions for cases involving extraordinary circumstances, preexisting nonconforming units, and situations allowing unions to combine two or more of the eight specified units into one. The American Hospital Association (AHA) (plaintiff) brought an action challenging the rule’s facial validity. The court enjoined the rule’s enforcement on the ground that Congress directed the board to avoid unduly proliferating bargaining units in the health care industry, but the appellate court reversed and upheld the rule. Noting the case’s importance, the Supreme Court granted review.