This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 蔎 is uncertain. It combines the grass radical 艹 with 設, but the original semantic and phonetic roles are unclear.
The grass radical 艹 sits above 設, which means 'establish' or 'set up'. Imagine setting up a fragrant herb garden, but the exact meaning is uncertain.
For セツ, imagine a set of fragrant herbs: set -> セツ, tying the reading to the idea of an arrangement.