What is the difference between the exhaustion requirement and the exhaustion rule?

^30. As we have explained, our procedural rules are designed to regularize the postconviction review process, upholding the finality of judgments while leaving open a safety valve for the presentation of legitimate claims. By contrast, the federal exhaustion requirement "is grounded in principles of comity and reflects a desire to 'protect the state courts' role in the enforcement of federal law,' [citation]. In addition, the requirement is based upon a pragmatic recognition that 'federal claims that have been fully exhausted in state courts will more often be accompanied by a complete factual record to aid the federal courts in their review.' [Citation.] Codified since 1948 . . . , the exhaustion rule, while not a jurisdictional requirement [citation], creates a 'strong presumption in favor of requiring the prisoner to pursue his available state remedies.' " (Castille v. Peoples (1989) 489 U.S. 346, 349, fn. omitted.)