It's the on /off socket for the tractor area meter. If you have a trailed implement that lifts out of work on a ram you can connect an on / off switch to said machine to work in conjunction with the ram movements. A two core wire fitted with a jack plug then fits into the socket you ask about and switches area meter on and off each time the ram is activated lifting machine in / out of work. It be useful with things like trailed mowers, beet / potato harvesters, de-stoners etc. We once rigged one up on a de-stoner and used a foot brake light pull switch from a motor bike.