VMS Help SPAWN, Qualifiers, /WAIT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /WAIT (default)
       /NOWAIT

    Requires that you wait for the subprocess to terminate before
    you enter another DCL command. The /NOWAIT qualifier allows
    you to enter new commands while the subprocess is running.
    (Use the /OUTPUT qualifier with the /NOWAIT qualifier to avoid
    displaying both parent and subprocess output on the terminal
    simultaneously.)

    Note that specifying the /NOWAIT qualifier causes both input and
    output to be shared with the parent process. If the input device
    is a terminal, control characters, such as Ctrl/T or Ctrl/Y,
    also affect all subprocesses sharing the input device. The Ctrl/Y
    control character, for example, interrupts all such subprocesses.

    This problem may be avoided by specifying /INPUT=NL:.