A transect is a profile of the intersections created by the transect object (a line, arc or traverse) and the triangles (TIN) generated by the surface. When you create a transect, you add the computed intersections to a new or existing traverse. Once added, the traverse can be viewed as a profile, showing the transect of the surface.
You’ll find all the instructions for creating a transect in the Contour Transect command.
When you transect a line or arc, TPC computes the intersection of the line or arc with the triangle edges in the TIN. A point is inserted into the transect traverse for each intersection.
When you select a traverse as the transect object, things work a little differently. Instead of computing the intersections of the traverse and the triangles, TPC computes the elevation of each traverse point that lies within the surface and places a copy in the transect traverse and updates the elevation of the copy to match the elevation of the surface at that point. You end up with a transect traverse that profiles the traverse based on the surface.