Allylic substitution occurs when a nucleophilic Lewis base displaces a nucleofugal Lewis base by attacking a remote (but "allylic") carbon centre. It is necessary that the propene function be substituted (labelled) in some way so that it is possible to tell if direct or indirect nucleophilic substitution has occurred. For more information look in the Chemogenesis
webbook sections on substitution
reactions and substitution,
transfer, abstraction, displacement (STAD).