Library /sys$common/syshlp/mailhelp.hlb SPAWN, Qualifiers, /WAIT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /WAIT
       /NOWAIT

    Controls whether the system waits until the subprocess is
    completed before allowing more commands to be specified. The
    /NOWAIT qualifier allows you to specify new commands while the
    specified subprocess is running. If you specify /NOWAIT, you
    should also use the /OUTPUT qualifier to direct the output stream
    to a file, rather than displaying it on your terminal. Doing so
    prevents your terminal from being used by more than one process
    simultaneously.