Fipronil (CAS-120068-37-3) in bulk form. The subject product, Fipronil, is a broad spectrum insecticide that belongs to the phenylpyrazole chemical family. It works by disrupting the central nervous system of the insect. Per your letter, Fipronil is also the active pesticide ingredient of a topical application used to prevent fleas, ticks and etc. İn companion animals.
[1] Please importantly note that only the relevant portion of the related is displayed above. You may refer to the said customs ruling for the entire ruling explanation.
[1]. https://rulings.cbp.gov/ruling/N081299
Fipronil (ISO) (CAS RN 120068-37-3) with a purity by weight of 95 % or more for the use in the manufacture of veterinary medicine
[1][1]. EU TARIC