Which Way

Do you know what's the reason behind it?

@richardfu_ yes,as @Frenzies said.

The 16 x 16 x 16 design constraints? I think I've run into it once or twice and I've seen several tutorials or demos that address it. The effects that it has on everything are starting to make sense now. It is almost as clear as mud.

@Lianne Malone When put a stair at height of 16 blocks, B cannot pass from three direction at the floor.