Does the venue for challenging the Environmental Protection Agency's (EPA) denials of small refinery exemption petitions lie in regional Circuits or the D.C. Circuit? Specifically, do these denials constitute "nationally applicable" actions or locally applicable actions "based on a determination of nationwide scope or effect" under 42 U. S. C. § 7607(b)(1)? The Court must determine if EPA's use of uniform statutory interpretations and economic theories to reach presumptive denials triggers the D.C. Circuit venue exception.

• Argued that its omnibus notices aggregating similar petitions into joint resolutions constituted "nationally applicable" actions under 42 U. S. C. § 7607(b)(1).
• Contended that any agency action affecting more than one Circuit is nationally applicable, as the statute refers to review in "the appropriate circuit."
• Asserted that if the actions were locally applicable, they fell within the "nationwide scope or effect" exception because they relied on a uniform statutory interpretation and economic theory.
• Argued that the term "determination" in § 7607(b)(1) suggests a resolution of an unsettled issue rather than the application of established policy.

• Argued that the denials were merely locally or regionally applicable because their legal effect was limited to the specific petitioning refineries.
• Contended that "determination" is a term of art that applies only when a Clean Air Act provision textually directs the EPA to make a determination for the entire Nation.
• Asserted that the "nationwide scope or effect" exception cannot apply because the EPA conducted confirmatory reviews of refinery-specific facts before issuing denials.
• Argued that the actions were not based on determinations of nationwide scope or effect since the final adjudication depended on individual circumstances.

• The Court held that while the denials are locally applicable, they fall within the "nationwide scope or effect" exception under 42 U. S. C. § 7607(b)(1), requiring venue in the D. C. Circuit.
• To identify the relevant "action," courts must look to the authorizing statute rather than agency packaging; thus, each denial under 42 U. S. C. § 7545(o)(9)(B)(i) is a distinct action.
• An action is "nationally applicable" only if it applies "[o]n its face" throughout the country (*Sierra Club v. EPA*, 926 F. 3d 844); because each denial applied only to one refinery, they are paradigmatically "locally or regionally applicable."
• The Court established a two-part test for the "nationwide scope or effect" exception: (1) the action must be "based on a determination of nationwide scope or effect," and (2) the EPA must find and publish that such a basis exists.
• The Court defined "based on" as requiring that the determination "lie[s] at the core of the agency action" and serves as the primary driver, rejecting a simple "but-for" causation standard (*Texas v. EPA*, 829 F. 3d 405).
• Courts must evaluate whether an action is "based on" such a determination de novo, making an independent assessment of the agency's reasoning (*Texas v. EPA*, 829 F. 3d 421).
• The Court found that EPA's interpretation of "disproportionate economic hardship" under § 7545(o)(9)(B)(i) and its "RIN passthrough theory" were determinations of nationwide scope or effect because they applied generically to all refineries.
• These determinations were the primary drivers of the action because EPA used them to reach a presumptive resolution to deny all petitions, rendering the subsequent review of refinery-specific facts "merely peripheral" (*Texas v. EPA*, 829 F. 3d 419).