Triangle Facing Direction
In order to achieve the best possible performance at runtime, Unreal Engine culls any triangles that it doesn't expect to be visible. This avoids spending any GPU cycles to render those triangles. In particular, the Engine assumes that all objects have a thickness. Any triangles that face away from the camera are assumed to be the back side of that object, and will be hidden from view by the front side of the object. This is called back-face culling, and it's an optimization technique that is used extensively in 3D rendering.
However, some design applications do not make the same distinction between the front and back faces of a surface, and may render surfaces regardless of which side you view them from. When working in your design tool, you may not always be aware of the facing direction of a given surface. This can lead to the surface normal, which is the direction perpendicular to the surface, pointing inward, or away from the direction that you need the surface to be viewed. Or, this may lead you to create planar, single-walled geometry without any thickness at all.
After you import your scene, this may cause some parts of your scene to be invisible when viewed from certain angles in the Unreal Engine. It may even look as if those parts of your model were not imported; however, the likelihood is that their surface normals are simply pointing away from the camera you're using to view the scene. If you rotate around the model, you may find that those surfaces suddenly appear.
For example, in the following scene (on the left), the ceiling and the ends of the pipes appear to be missing, even though the geometry was imported correctly. Enabling the Two Sided option for the Materials makes the surfaces appear (see below for details).