SI is useful when shallow, cool air below 850mb conceals grater convective potential aloft. SI is useful when tornadic potential is high, because the moist layer almost always extends above 850mb during violent outbreaks. These are the defined levels.
Showalter Index greater than 0: Stable, but weak with convection possible for SI = 1-2 if strong lifting is present.
Showalter Index between 0 and -3: Moderately unstable.
Showalter Index between -4 and -6: Very Unstable.
Showalter Index between less than -6: Extremely Unstable.