surrogate, act in addition to
攝 centers on acting in place of another or supplementing an existing role: a surrogate who takes over duties, or someone who acts in addition to the main person.
攝 is the traditional form of 摂. It combines 扌 (hand) with 聶, which likely contributes the sound and suggests gathering or taking in, related to the idea of taking over a role.
The left hand 扌 reaches out to take over the duties represented by the complex right side 聶, like a surrogate stepping in to manage things.
For セツ, imagine a surrogate sets up a temporary office: sets -> セツ, and the hand takes charge.