The case was brought about by Marine James Kiso who served in the Vietnam War. He sought benefits for Post Traumatic Stress Disorder. The Department of Veterans Affairs in 2006 agreed that Kiso did, in fact, have PTSD. The VA refused to provide Kiso with backpay for benefits dating back to 1983, when he initially filed his claim.
From SCOTUS Blog:
The VA’s denial in Kisor’s case hinged on its interpretation of the term “relevant” in one of its regulations. On appeal, the U.S. Court of Appeals for the Federal Circuit ruled for the VA. The court of appeals acknowledged that the regulation was ambiguous, but it followed the Supreme Court’s cases – Auer v. Robbins and Bowles v. Seminole Rock & Sand Co. – instructing courts to defer to an agency’s interpretation of its own regulation unless the interpretation is plainly wrong or inconsistent with the regulation.Multiple amicus curiae briefs were filed in this case.
The Center for Constitutional Jurisprudence, an arm of the Claremont Institute, argued about the dangers of an agency interpreting their own regulations:
Granting deference to the agency to interpret its own ambiguous regulation cedes the judicial function to the Executive. This is an invitation to agencies to avoid the expense and bother of rulemaking proceedings when it wants to change its policy. Instead of going through the process to allow public participation and judicial review of the change, it can instead merely change how it interprets its existing regulations.
Denying “controlling deference” to an agency interpretation does not mean that the courts must ignore long-standing agency interpretations and practices. Those remain important interpretative tools. Yet the job of interpreting the legal text will remain with the courts. To do otherwise results in a failure of the duty of the judicial branch of government “to declare the sense of the law” and thus violates the separation of powers required by the ConstitutionThe Chamber of Commerce of the United States' explained why businesses have a vital interest in this case:
Auer deference harms the business community by encouraging agencies to adopt vague regulations that they can later interpret however they see fit. This practice upsets the expectations of regulated parties without the notice provided through formal rulemaking. When agencies adopt vague regulations, businesses must attempt to predict how the agency will interpret those regulations and also how likely the agency is to change that interpretation in the future. Businesses also have a more difficult time tracking an agency’s shifting interpretations. Regulated companies cannot learn of changes to their regulatory obligations simply by reading the Federal Register because the agency is just as likely to change its interpretation of a vague regulation by, for example, filing an amicus brief.Other amicus curiae filers include the National Association of Home Builders, American Farm Bureau Federation, the National Association of Manufacturers, the National Cattlemen’s Beef Association, the National Mining Association, the Southeastern Legal Foundation and the Beacon Center of Tennessee as well as the State of Utah.
The real win, however, would be for gun owners. Specifically, if Congress decides to rule in Kisor's favor, it means the Department of Alcohol, Tobacco, Firearms and Explosives (ATF) could be reigned in. Right now, there are legal gray areas as far as implementing laws go.
LKB at The Truth About Guns hit the nail on the head:
However, in most cases, Congress leaves it to federal agencies to implement those laws, including promulgating the regulations that govern the nuts-and-bolts of how the law will works.
For example, the National Firearms Act of 1934 imposed a tax on the manufacture and transfer of machine guns, suppressors, SBS/SBR’s, etc., and required them to be registered with the federal government. However, the ’34 Act didn’t specify how this would be done (e.g., would it be something that you could just go and do on a cash-and-carry basis at your local post office, or did it require all of the hoops we see today?).
Congress left it up the bureaucrats to come up with the regulations on how to implement that law. To a certain extent, that makes sense; it’s impossible for every act of Congress to include every detail of how each law is to be implemented.
Of course, many of these regulations themselves wind up being cumbersome or hard to understand, especially when they deal with technical matters or may conflict with other laws or regulations. Thus, the same agency that promulgated the regulations is often required to interpret them, both formally (such as in a decision in an administrative enforcement proceeding) and informally (e.g., in an opinion letter).It looks as though Kisor – theoretically – should have the votes to win his case, especially with the addition of Justices Kavanaugh and Gorsuch. If that does happen, we could see a roll back in some of the red tape we see, especially when it comes to the ATF's interpretation of laws.