The first texture setting controls when a photograph is valid as a source of texture for a particular part of a surface. These values apply when the Texture setting in the Materials Dialog is set to Multiple-Photo Texture.
A surface normal angle setting of 80 degrees says this, “if a part of a surface has a normal (the perpendicular direction away from the surface) that subtends an angle with the light ray from the photo to that part of the surface that is 80 degrees or less, then that photo is a valid source of texture for that part of the surface”. So for example if you had two surfaces in the model and one pointed towards photo 1 (an angle of 20 degrees let’s say) and the other was at a very sharp angle to photo 1 (an angle of 90 degrees (so the photo 1 camera is seeing the surface edge-on)) then photo 1 will be used as a source of texture for the first surface but not the second.
If a surface does not face any photo within this angle tolerance then it will get no textures. If a surface faces more than one photo within this tolerance then the photo textures will be blended based on the Multiple-Photo Texture Blending Factor control.
The setting “When a surface normal points away from all photos in the set _ try flipping the normal 180 degrees” controls what to do when there is no appropriate photographic data to texture a surface or part of a surface. This would happen when no photographs were taken at a suitable angle to that part of the model. If no texture is available then the surface normal is rotated 180 degrees and the photos are checked again as possible sources of texture. This can produce some strange results but in some cases with symmetric objects and simple textures the result can look better than no textures.