These are hard to answer in the abstract - where the true answer will depend on the actual facts and circumstances (which we don’t have here). If a chat app is standalone and free, but built in UE4, there is no royalty. If a chat app is standalone and not free (and built in UE4), there is a royalty on the chat app revenue. If the chat app is not standalone but rather integrated into a larger product, that makes the larger product a Product under the EULA.