Think of it as how light is traveling. More Positive will scatter more towards the direction it was traveling. Zero will scatter it evenly. Negative will scatter back towards the light. If your scene is pretty evenly lit you won’t notice a difference. This could be used to create white smoke or brightly colored smoke. Side note: When using a colored volume the closer you get to +1 the more you will see complementary color, because of the light bounce.
