VMS Help DECTPU, TPU$SPECIFY_ASYNC_ACTION, Argument  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 facility_index

    OpenVMS usage:longword_unsigned
    type:         longword (signed)
    access:       read only
    mechanism:    by reference
    Represents an index of the asynchronous action. This index is
    used with the TPU$TRIGGER_ASYNC_ACTION routine to let DECTPU know
    what action to perform. It may also be used to delete an action
    routine (by omitting the tpu_statement). You may register several
    asynchronous actions depending on your application's needs. This
    facility index number may be any positive integer.

 tpu_statement

    OpenVMS usage:char_string
    type:         character string
    access:       read only
    mechanism:    by descriptor
    The DECTPU statement you want executed when you call the
    TPU$TRIGGER_ASYNC_ACTION routine. The statement is compiled and
    then stored internally. If you omit the parameter, DECTPU removes
    the action from its list of asynchronous events.