Hi,
As a pure guess, perhaps it's the theoretical neutral point in the suspension movement but honestly I've no idea.
I modified the drop links to be adjustable ages ago and have been running with the bar just below the closing plate. I've also got slightly lowered suspension so if you did the same mod I would check that the increased angle doesn't distort the mounts on the dampers too much.
This is how mine looked when I first did it. Hard to tell from the photo, but it's fairly close to the plate.
Brian