The spindle is the part that the front axle attaches to.
The spindle is attached to the caster block and has room for 2 spacers so you can either have it top, middle or bottom. As you are adjusting the height of the steering knuckle in relation to the bellcrank (by changing it to top/middle/bottom), you will need to keep an eye on bumpsteer. You will potentially need to add/remove washers under one end of the steering, depending on how you adjust the spindle height.
For example, spindle low would mean both spacer go at the top (between the spindle and caster block).
HTH
