seal radical (no. 26)
卩 is a radical character that represents a kneeling person, often associated with seals, stamps, or formal kneeling postures. Its standalone meaning is tied to this radical sense.
卩 is a pictograph of a kneeling person, historically used to represent a seal or stamp. The modern form preserves the basic kneeling shape.
Picture a person kneeling down to press a seal into a document: the curved stroke is the bent knee, and the vertical line is the upright posture.
For セツ, imagine a kneeling person setting a seal: set -> セツ, and the seal is pressed down.