SDO = Selective Door Opening. Basically, allows some carriage doors to be opened, but not all (for short platforms). It's not something I've any experience with, but basically, I think it allows doors to be opened from the active panel to all doors to the rear, or the active panel to all doors to the front.
Scotrail use 'One Door Operation' for short platforms instead - basically, the Conductor will open their 'Local Door' and that's it. There's definetly less scope for incidents, although it's only really suitable for stations with small traffic levels (Dalwhinnie springs to mind). It's used at Beauly, Conon Bridge, Attadale & Duncraig (request only for both). Possibly some on the West Highland Line too, I think?