/sys$common/syshlp/helplib.hlb RUN, Process, Qualifiers, /MAILBOX  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /MAILBOX=unit

    Specifies the unit number of a mailbox to receive a termination
    message when the created process is deleted. If no mailbox is
    specified, the creating process receives no notification when the
    subprocess or detached process has been deleted.