Thick question time, what does "hold" do?
I was waiting for that...!!

Not thick at all, rocket science in fact!
Briefly; when the doors are locked on an HST, the panel used sends a continuous 'lock signal' around the CDL circuits - a bit like switching on a light. This fails safe, so in the event of a power failure the CDL will fail unlocked, rather than trap everyone on board. When you then unlock, the panel stops sending this 'signal' and the doors unlock. Simples. Only, it was designed always to have the whole train locked and unlocked, before SDO. So it didn't matter where on board you unlocked from, because the whole train was being opened that previous panel sending the signal would also be unlocked, and thus would stop sending the signal.....are you with me?!
SDO introduced a problem. Let's say you locked up from the rear coach at Oxford, and the next stop is Hanborough which requires only the front two coaches to be unlocked - that last panel at the rear will continue sending the lock signal, even while you're operating the SDO elsewhere, because it isn't being unlocked and the signal isn't being cancelled. This effect can be overridden whilst you have a panel active elsewhere, (keys in, panel in Doors Ahead/Behind), but as soon as the Guard turns the panel off to remove his keys and venture onto the platform, that other panel kicks back in and all the doors lock again. So, 'Hold' was devised, which essentially keeps the second panel active but allows keys to be removed, meaning that panel continues to override the lock signal being sent from elsewhere.
Are you still with me?!
It works well, but it's restrictive in that the Guard has to return back to that panel to lock up again, because it is still live and no other panel can therefore be used. Sometimes it's unavoidable, but the general approach is always to try and lock using a panel which will be within the part of the train being unlocked next time, thus avoiding the issue
