A more thorough explanation:

Marks rule is a legal doctrine that applies when the U.S. Supreme Court issues a fractured, plurality opinion. In this situation, the opinion of the justices who agree on the narrowest grounds is considered the Court's holding. This means that the legal standard with which a majority of the Court would agree becomes the official decision.

For example, if the Supreme Court hears a case and issues a 5-4 decision with three different opinions, the Marks rule would apply. The opinion that is supported by at least five justices and has the narrowest legal reasoning would become the official holding of the Court.

The Marks rule was established in the case of Marks v. United States in 1977. In that case, the Court had issued a fractured opinion with no clear majority, and the Marks rule was used to determine the official holding.