Some vehicles (mostly off-road) allow you to move the axle (and steering block) up or down a certain distance from center. This is achieved by placing spacers either above (to lower the axle) or below (to raise the axle).
Changing the height of the Axle can produce different handling characteristics.
Raising the Axle (putting the shims under the steering block) will give the car less front grip entering corners and a little more grip on exit.
Lowering the Axle (putting the shims on top of the steering block) will give the car more front grip entering corners, but a little less grip on exit.
Keeping the Axle in the middle position (one shim above and one shim below the steering block) is best for most conditions. It will give the best feel of enter and exit steering.