In some cases it is useful to examine parts in a selected view angle. For example, when you reinforce concrete elements, you can easily check the distances between the reinforcing bars.
Tekla Structures displays the part in the selected view angle. The view angle is based on the selected part's coordinate system so that the top view is looking to the negative z-direction. For example, if you select the top view, Tekla Structures sets the current view angle from up to down in the coordinate system.