For fn:Ignore() (no argument) it means that the PDF spec explicitly defines the key (and possibly some other data integrity rules) but then also states it should be ignored or words to that effect ...

230410 Predicate Logic Discussion Questions v1.md 230410 Predicate Logic Discussion Questions v1.pdf 230502 Predicate Logic Reading v1.md 230502 Predicate Logic Reading v1.pdf 230503 Predicate Logic ...