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https://forum.unity3d.com/threads/_worldspacelightpos0-how-it-works.435673
The gradient map rotates according to the rotation of directional light. Texture RGB (but not Alpha) also multiplied by directional light intensity, meaning the cloud gets darker when the light is dimmed. bgolus provided the most crucial part of the shader codes.

bgolus has also clarified that, while it looks alike, the method used here is different than the billboard volumetric smoke particles seen in Shadow of the Colossus (PlayStation 2).

According to Unity technician Richard Kettlewell, having particles receiving light from point light or spot light is performance taxing because of the additional calculation which isn't present with directional light.