A geometric primitive is exploded to a TC surface, as John says, to individually edit vertices. When the surface object is selected with the edit tool, each of its vertices are marked with a blue box. The vertices in a field selected for moving will turn magenta. You can pick up one vertex at a time by clicking on it, or more than one by fencing them or shift selecting them, then clicking on one, the others will follow. The node you've picked up will be the snap point, you have to put them down and reselect a new group or change pickup vertex to snap another. Vertices snap in the same way as selection reference points.
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