Pick a Tool. 2D slices: left-drag (Pen/Eraser paint, Fill floods a closed loop, Vector draws a polygon) or click (Magic wand grows an intensity-similar region; 2D confines it to the slice).  |  3D exploded blocks: right-drag (Pen/Eraser/Vector) or right-click (Fill floods the blob, Wand grows the region) directly on a block. Explode must be on.  |  SVG exports vector annotations. Clip shows a cutaway from its own control (right-drag keeps drawing); Alt+right-drag rotates the clip plane.