In 3ds Max, faces are one-sided. The front is the side with the surface normal. The back side of the face is invisible to the renderer; meaning the face appears to be missing when viewed from the back. Objects are usually created with the surface normals facing outward, but it is possible to create objects with the faces flipped or to import complex geometry in which the face normals are not properly unified.

Rendering of a box with a double-sided material, and same box with a single-sided material

There are two ways to render both sides of a face. Either turn on the Force 2-Sided option in the Render dialog, or apply a two-sided material to the geometry.

Usually, you want two-sided rendering turned off since it slows rendering time. However, if you want to render the inside as well as the outside of objects, or if you've imported complex geometry (for example, from AutoCAD) in which the face normals are not properly unified, you can use one of the preceding methods to render each face regardless of its normal's orientation. You can also unify normals explicitly by using the Normal modifier.