Bump / normal map
Normal mapping and bump mapping are techniques meant to achieve the same goal: deform the surface of an object. So why not use the same slot for both? When uploading a texture, the material editor detects the color and automatically assigns the right type for you.
Polygons located on a mesh have a front and a back side by default. Therefore, wherever you look you can see something.
However, in some cases you may want to disable this characteristic by unchecking the “Double sided” material option and display only front faces, making the back side invisible.
You can use this option with transparent objects where back and front faces are drawn in an unpredictable order, for example.
While it’s not realistic to draw only the front faces of a transparent object, it definitely gives a nicer result.