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

    A character string containing the system cluster identification
    number in the system's list of clustered nodes. If the current
    system is not a member of a cluster, the first return value is
    null. After the last system cluster identification number is
    returned, the F$CSID function returns a null string ("").