5. Almost any legal rule, whether labeled procedural or substantive, has the potential to affect the outcome of litigation. Does this mean that, after York, a federal court in a diversity case must apply every state legal rule that, if enforced, would affect the outcome of litigation?

Does this include housekeeping rules like rules about the size of briefs or the color of paper used? Did Justice Frankfurter mean any rule that could affect the outcome, or would he say that only rules that influence a lawyer’s choice of forum at the time she is choosing a forum are “outcome determinative”?