The carrier with the ring gear on it is suspended in the case between a pair of cup and cone bearings which mate into the side plates. You are squeezing the bearings that the carrier hangs in between the side plates so how far you screw in the side plates determines both the bearing preload and the ring gear's positioning left and right in the case and relative to the pinion.
There is a special Subaru tool for turning the side plates. It would be very near impossible to properly set up a Subaru front differential without this tool. You are actually putting quite a bit of torque on the side plates when you are determining the zero point (left/right placement of the ring gear) and setting the bearing pre-load.
Before touching the side plates I mark them and the case with a black marker so I can compare the current adjustment to changes that I make.
Driver's side (LH) side plate removed:
Repeat on passenger side:
You should be careful while doing this not to damage the plastic gears that drive the speedometer. I screw the first side plate back in partially while I flip the case over and loosen the other side so that I can more carefully take things apart. You don't want the carrier/ring gear banging around breaking the plastic gears while you flip the case.