During pattern matching, the AbsentLink can be used to indicate absence. Although AbsentLink is conceptually equivalent to (NotLink (PresentLink ...)), in practice, the NotLink cannot be used in this way, as it requires a negative confirmation, which is hard to do. There are some deep theoretical reasons for this; see in particular the Wikipedia article as well as the Stanford article on intuitionistic logic.
See also this page for some more information.
In PLN NotLink corresponds to the negation of a concept or a predicate, formally
NotLink <TV> A
Where denotes the function associated with the fuzzy connector , usually .