When you rig a parameter, you usually rig the Minimum and Maximum values you want to have used. To do so you click on the small circles/dots under the Slider bar (they're called "snapshots") -- they will highlight blue when they're active. Once you activate dot, then you use the parameter controls you attached to the rig and set the value they should be at that point. Then activate the other dot and set what it's value should be at that point. When the user uses the rig, the slider will automatically interpolate between the two values you set for the dot.
If you turn down the disclosure triangle at Options, you will see you can set the Slider's Range Minimum and Range Maximum values (what the user sees in FCPX - so you can set -100 to 100 or a range from 50 to 60 if you wanted to - basically anything you like.) The important feature in the Options is the Interpolation. The default is Linear, so when the user slides from the minimum to maximum value, the parameter values will change in a linear manner. You also have the option to have the values Ease (out/in between snapshots) or you can use Constant. Constant keeps the same values across all rigged parameters until a new slider snapshot is crossed. This means you can add as many snapshots as you want and control how values progress over the range you set.
You add snapshots by double clicking under the slider bar. You can click on the dot and drag it to any position you like under the Slider bar (absolute precision setting the locations is next to impossible, so don't knock yourself out trying to get exact positioning at locations like 0.25, 0.5 and 0.75 -- it can be done, but you need to widen the inspector pane out as far as you can stretch it and it still takes several attempts to get the values to line up... it CAN be done... you just have to figure out if it's that worth it.)
Anyway, by adding several snapshots and setting their Interpolation to Constant, you can set up "jumps" from value to value -- you don't have to have a linear progression from your minimum to maximum.