It's so you can measure rear caster on the suspension... it's just a purpose built device to measure caster and goes into two locating holes in the knuckles that are already there from the factory. (Digital Angle Gauge CH-47960 released by SPX Kent-Moore) The sky/sol use front and rear unequal length a-arms with coilover shocks from the factory. Everything is more or less infinitely adjustable, except for caster which ends up being a product of how much negative camber you dial in (as you add camber, you lose max caster) but you still want as much negative caster in the rear as possible without the two sides l/r being uneven.