Note: Your second case is correct if it’s a work-for-hire or consulting situation, but if you were selling the same visualization app for $10k each to multiple different companies then I think it might be subject to royalties at that point (but IANAL). When in doubt, the EULA (Game-Engine-Technologie von Unreal - Unreal Engine) is the definitive legal&binding agreement, there isn’t a separate document later on to execute or anything like that.