[Haskell] reasons for non-portability

Hello, according to http://www.haskell.org/hierarchical-modules/libraries/ reference-libraries.html#MODULE-HEADER, each module should have a header which contains a line about portability. My question is, if a module is considered non-portable only if it contains non-portable constructs itself, or if a module is also non-portable if it just imports a module which is non-portable. Wolfgang