VMS Help Lexicals, F$IDENTIFIER, Return Value  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    An integer value if you are converting an identifier from a name
    to an integer. The F$IDENTIFIER function returns a string if
    you are converting an identifier from an integer to a name. If
    you specify an identifier that is not valid, the F$IDENTIFIER
    function returns a null string ("")  (if you are converting from
    number to name) or a zero (if you are converting from name to
    number).