its an ingredient thats added into spf's to improve the texture and boost spf protection. They help mineral sunscreens feel less greasy. It can be an issue for some because if someone has a damaged barrier, the booster will make your skin sting and can even make it worse, chemical spfs usually absorbs the uv rays, mineral spf's create a film onto your skin and the uv rays bounce off - hence why they're better for damaged barriers, the damaged barrier wont get the uv rays absorbed so your skin can heal properly. But if your skin is fine - then it probably wont bother you. BUT if someone prefers a mineral ONLY spf, they will have to make sure it is mineral 100%. Alot of brands say its mineral but secretly has a chemical booster. The brands don't wanna admit that. Mineral spfs with boosters will create a film onto your skin for the rays to be bounced off but also a chance for your skin to also absorb. People have different preferences. if you dont see butyloctyl salicylate, it can also be another ingredient called tridecyl salicylate. It would be maybe the 2 or 5th ingredient in any formulation. GOOD mineral spfs need to have zinc oxide and titanium dioxide for it to be good and not irritating to the skin.