Dear Abdullah Eidani, a 3D Shell is usually a FE element that can be used in 3D, i.e. 3D displacement, Motion, Deformation, but keeps it Shell properties (i.e. a rather thin and constant thickness at every Position of the Shell shape). Hence a 2D planar Shell can only be used in 2D Environment. Hence 2D elements allow for tube-section or cylinder-section Deformation or for planar element representation (flat or rectangular Arrangements). The most simple one and, hence most restricted element is the planar Shell element.