/sys$common/syshlp/helplib.hlb System Services, $CLREF, Argument  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 efn

    OpenVMS usage:ef_number
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Number of the event flag to be cleared. The efn argument is a
    longword containing this number; however, $CLREF uses only the
    low-order byte.