variety of marsh grass
辥 refers to a specific type of marsh grass, a plant growing in wet, swampy ground.
The exact historical development of 辥 is uncertain. It likely combines a plant-related left side with the phonetic component 辛 on the right, but the original form and meaning evolution are unclear.
The left side looks like a tangled mass of grass, and the right side 辛 (spicy, bitter) suggests the harsh, bitter taste of marsh plants. Picture a clump of bitter marsh grass growing in a swamp.
For セツ, imagine cutting (切る, せつ) through a thick patch of marsh grass with a sickle: 切 -> セツ, and the blade slices the grass.