In Heckler v. Chaney, the decision not to bring an enforcement action is equivalent to the decision not to adjudicate something. On the other hand, in Mass v. EPA, the decision about whether to engage in rulemaking is generally judicially reviewable unless the 2 APA restrictions apply. If someone petitions an agency to make a rule, the agency has to be able to justify the grounds for denial if the person requesting rulemaking petition is an interested party connected to the issue. (APA 555(e))