/sys$common/syshlp/helplib.hlb Lexicals, F$PID, Argument  *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 processes. The F$PID function uses this pointer
    to return a PID.

    Specify the context symbol by using a symbol. The first time you
    use the F$PID function in a command procedure, you should use
    a symbol that is either undefined or equated to the null string
    ("") or a context symbol that has been created by the F$CONTEXT
    function.

    If the context symbol is undefined or equated to a null string,
    the F$PID function returns the first PID in the system's process
    list that it has the privilege to access. That is, if you have
    GROUP privilege and if the context symbol is null or undefined,
    the F$PID function returns the PID of the first process in your
    group. If you have WORLD privilege, the F$PID function returns
    the PID of the first process in the list. If you have neither
    GROUP nor WORLD privilege, the F$PID returns the first process
    that you own. Subsequent calls to F$PID return the rest of the
    processes on the system you are accessing.

    If the context symbol has been created by the F$CONTEXT function,
    the F$PID function returns the first process name in the
    system's process list that fits the criteria specified in the
    F$CONTEXT calls. Subsequent calls to F$PID return only the PIDs
    of those processes that meet the selection criteria set up by
    the F$CONTEXT function and that are accessible to your current
    privileges.