A Manual Termination Waypoint (MTW) or Downwind Termination Waypoint (DTW) usually ends in a heading requirement so initially you may use managed descent (DES) until you get to this leg as the FMGS has something to plan to go to and an altitude to descend to. On the MTW leg, because you are probably going to take up a heading at some stage, you may be left with a non-managed part of the leg and therefor DES (managed) would not be sensible as there may not be sufficient information/distance etc for the FMGS/autopilot to complete the manoevre. It is saying that you should plan to descend either in 'Open Descent' mode or select Vertical Speed mode so that you have control of the flight path manually.
If you have had prior clearance to proceed from the DTW to intercept the localiser and are doing so in a managed mode then of course you can select Managed DES and continue fully managed.
