leash
緤 centers on a physical tether that restrains or connects: a leash used to hold an animal or, by extension, a bond that ties things together.
緤 combines the thread radical 糸 with the phonetic component 枼, suggesting a cord or leash. The exact historical development is uncertain, but the phono-semantic structure is typical.
The thread radical 糸 on the left represents a cord or rope. The right side 枼 looks like a wooden frame with a crossbar, like a post to which a leash is tied. Together, they form a leash made of thread or rope.
For セツ, imagine a leash made of strong sets of threads: sets -> セツ, and the leash holds the animal securely.