Scenes can be loaded either to replace the current scene or can be loaded additively to add its contents to the current scene without unloading the preceding one. This can be toggled via the LoadSceneMode argument of the SceneManager.LoadScene() family of functions.

Another mode of scene loading is to complete it either synchronously or asynchronously, and there are good reasons to use both. Synchronous loading is the typical means of loading a scene by calling SceneManager.LoadScene(), where the main thread will block until the given scene completes loading. This normally results in poor user experience, as the game appears to freeze as the contents are loaded in (whether as a replacement or additively). This is best used if we want to get the player into the action as soon as possible, or we have no time to wait for scene objects...