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Deform is a trickier feature than the others. It has three top–level options. By far the most useful of the three is the Curve To Curve deform, which enables you to deform one body to match another using a start and a finish curve. A second Deform option is the Point deform, which enables you to grab a point on a model and globally deform the entire model, based on how you move that one point. A third Deform option is the Surface Push, which vaguely works like the Indent feature, but completely lacks all precision.
Deform first requires some experimentation to figure out what it can do, and then it requires some imagination to see what you would do with it. I personally only see a real applicable use for the Curve To Curve option.