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

       /WAIT (default)
       /NOWAIT

    Controls whether the system waits until the subprocess completes
    before you can specify more commands. The /NOWAIT qualifier
    enables you to specify new commands while the specified
    subprocess is running. If you specify the /NOWAIT qualifier,
    use the /OUTPUT qualifier to direct the output to a file instead
    of displaying it on the screen. Doing this prevents your terminal
    from being used by more than one process simultaneously.