Twin electrics (the second socket is a 12S socket) provides you
with two 7 pin sockets at the rear of your vehicle. This allows
you to tow any trailer or caravan as your entire road legal lighting
(including reversing lights) are provided.
The grey or white 12S socket provides power to the caravan battery,
fridge, interior light and reversing lights and is optional when towing a caravan.
Many people who only tow a caravan for a couple of hours and
will then be pitching up at a caravan site with mains electricity
opt to do without the 12S socket and just fit single electrics to their vehicle