It's very much like the pick-up configuration on electric guitars.
A clavinet uses pick-ups that work in a similar way .Imagine a guitar with two pups. They are probably exactly the same as each other, but because of their positions, they produce different tones.A single coil pup will produce a thinner sound than a humbucker. When pups are in series (electrically) they produce a different sound from when they are in parallel.Or - in and out of phase.
So the AD, BC etc. are different configurations of the pups, changing the output sound. A little academic as when you find a sound that's good, you don't need to know what pup is switched, only that it makes the sound you like. Sort of 'tone controls' on switches. Obviously rotary pots on your equipment can still be used to adjust tone more.