Domains define the overall usage of a material. Most shaders will be Surface however, if a shader needed to be used as a decal, light function or post process, it has to belong to those Domains.
Is the default and is used for any kind geometry (static, skeletal, landscape, etc...).
If the material is to be used by Decal Actor, it needs to be set to this or it will not work.
Used by materials applied to lights as LightFunctions.
If the material is used as Blendable Material in a PostProcessing chain.
This material will be used for UMG or Slate UI