VMS Help PSM Routines, PSM$REPLACE, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The PSM$REPLACE routine must be called each time a user service
    routine replaces a symbiont routine or is added to a set of
    symbiont routines.

    The code argument specifies the symbiont routine to be replaced.
    The routine codes that can be specified in the code argument
    are of two types: those that identify existing print symbiont
    routines and those that do not. All the routine codes are
    similar, however, in the sense that each supplies a location
    within the print symbiont execution stream where your routine can
    execute.

    By selecting a routine code that identifies an existing symbiont
    routine, you effectively disable that symbiont routine. The
    service routine that you specify might or might not perform
    the function that the disabled symbiont routine performs. If
    it does not, the net effect of the replacement is to eliminate
    that function from the list of functions performed by the print
    symbiont. Exactly what your service routine does is up to you.

    By selecting a routine code that does not identify an existing
    symbiont routine (those that identify the input-filter and
    output-filter routines), your service routine has a chance to
    execute at the location signified by the routine code. Because
    the service routine you specify to execute at this location does
    not replace another symbiont routine, your service routine is an
    addition to the set of symbiont routines.

    As mentioned, each routine code identifies a location in the
    symbiont execution stream, whether or not it identifies a
    symbiont routine.