/sys$common/syshlp/helplib.hlb Lexicals, F$CSID, Arguments  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 context-symbol

    Specifies a symbol that DCL uses to store a pointer into the
    system's list of clustered nodes. The F$CSID function uses this
    pointer to return a cluster identification number.

    Specify the context-symbol argument by using a symbol. The first
    time you use the F$CSID function, use a symbol that is either
    undefined or equated to the null string.

    If the context-symbol argument is undefined or equated to a null
    string, the F$CSID function returns the cluster identification
    number of the first system in the system's cluster node list.
    Subsequent calls to the F$CSID function will return the cluster
    identification number of the rest of the nodes in the cluster.