Signifies that value of this property contains the main texture of the Material.
This flag corresponds to the [MainTexture] ShaderLab Properties attribute.
By default, Unity considers a texture with the property name name "_MainTex" to be the main texture. Use the [MainTexture] ShaderLab Properties attribute to make Unity consider a texture with a different property name to be the main texture.
Additional resources: Material.mainTexture, ShaderLab: Properties, ShaderPropertyFlags.MainColor.