Kind, Type, Seed
If a grain seems extra heavy, you know it's a different kind. Not all types of grains are the same. So when you feel the heavy type, you know it isn't the same as the light kind.
Imagine sorting all the different types of grains by their weight.
しゅ
たね, ぐさ
None
You sort all of the different kinds and types by putting them in the right chutes (しゅ). They slide away, probably to some big type-based piles you can't see.
Picture the two chutes you use to separate the different types of grains. One has a sign that says "Light Chute" and the other says "Heavy Chute."