Sd Alcohol 40-B

SD-Alcohol and some numer+letter refers to alcohol (or ethanol) being denaturated with different kind of additives. These additives make sure that the alcohol is poisonous and bad tasting so that nobody drinks their alcohol-loaded toner. 40-B specifically means the additives are denatonium benzoate and t-butyl alcohol.

As for Alcohol Denat in skincare, it's a controversial ingredient. It's a great solvent and makes cosmetically elegant, light formulas but at best it's skin-drying, at worst it's skin-barrier damaging. We have written about alcohol way more here.