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A Cone rating means that you can fire that clay at any temperature up to that cone. The closer you get to the maximum rated cone, the stronger and denser your clay will be. You cannot fire a clay higher than its maximum rated Cone, or it will melt.
Low-fire clay can only be fired up to Cone 04, or sometimes a little higher. Unlike mid and high fire, it never shrinks much or gets really strong and dense even when fired to its maximum temperature. The main advantage to using a low-fire clay when low firing is that your glazes may craze less than if you used a Cone 10 or Cone 6 clay.
As always, you have to fire your glaze to the Cone that is specified for that glaze, regardless what clay you use. Just make sure you use a clay rated at least as high as the glaze.