This covers apps that facilitate the sale of illegal goods (drugs, weapons, counterfeit documents) or coordinate other illegal activity — Google evaluates this against local law in each region the app is distributed to, not just a single global standard.
Often triggered by marketplace or classifieds apps with insufficient listing moderation, where prohibited items get posted by users faster than they're removed.
"Your app facilitates the sale or promotion of a good or service that is illegal in one or more regions where it is distributed, per the Illegal Activities policy."