Curved surfaces do not have all the view directions you can specify in the material definition. It is important to know that the faces are designated on curved surfaces to understand how surface hatching is applied when these objects are assigned material definitions.
An object with a vertically curved surface, such as a Barrel Vault mass element, has no front and back faces, as nothing could delineate them. The curved surface of the Barrel Vault mass element is considered to be the top face of the object.
Barrel vault mass element with material assigned to top face
An object with a horizontally curved surface, such as a Cylinder mass element, has no left and right sides, as nothing can delineate them. The curved surface is considered to be the front face.
Cylinder mass element with material assigned to front face
An object that has only one curved surface, such as a Sphere mass element, has only a front face.
Sphere mass element with material assigned to front face