Shader tags define the visual appearance of models in the game world. The shader tag class itself is abstract and cannot be created directly, but is extended by the various child shader types like shader_model and shader_environment.
Causes lightmap UVs for this material to be based on existing texture UVs rather than generated completely anew by Tool/Sapien. This can be used to fix squiggly or jagged shadowing artifacts often seen on large complex surfaces using a single material. Note that any overlapping UV faces may cause lighting artifacts because one part of the lightmap texture is being used to represent different locations.
Lighting will ignore surface normals (the direction faces point). Use this for shaders where the surface normals are not a good representation of lighting directionality, such as foliage mapped to flat planes for trees and bushes.
The surface will receive the worst possible quality of lightmaps. Use this for shaders which will be used in areas which do not need detailed shadows or the player is unlikely to visit in order to save texture space.