Specifies a volume to be used as the default volume for all
  succeeding commands.  If the USE command is not used, then
  the default volume is SYS$DISK.

  FORMAT:
  USE <device>

  Where <device> is some device spec. or a logical name that
  translates to a device spec.  The device must have some
  media MOUNTed on it.

  EXAMPLE:
   USE DBA1: