The Cut 2D Section command creates a cross-section, or 2D contour, on the cutting plane. The contour is created by the intersection of the model with an infinite plane passing though the section line. Only the elements that actually intersect the section line are shown.
The Cut 3D Section command creates a section with all the 3D geometry that remains on the indicated side of the infinite plane passing through the section line. The elements that intersect the section line, as well as the 3D geometry that exists beyond the line, are shown.