A game viewport (FViewport) is a high-level abstract interface for the platform specific rendering, audio, and input subsystems. GameViewportClient is the engine's interface to a game viewport. Exactly one GameViewportClient is created for each instance of the game. The only case (so far) where you might have a single instance of Engine, but multiple instances of the game (and thus multiple GameViewportClients) is when you have more than one PIE window running.