"Top of canopy" - view from above like looking down from an airplane window. You won't see any shadows looking down on a forest because you only see sunlight landing on the top of the canopy. Most accurate.

"Below canopy" - what shadows would look like on the ground. If you're in a dense forest everything is in shadow because the trees obscure the sun. I guesstimate how big and where the below canopy shadows land. Any vegetation that's more than 2 meters tall, I place a shaded area beneath it. Not super accurate because it treats trees like cylinders instead of a sphere/triangle on top of a cylinder.

"No trees" - only building and terrain shadows are shown on the map.