Used when opening a Scene in the Editor to specify how a Scene should be opened.
Additional resources: EditorSceneManager.OpenScene.
| Single | Closes all current open Scenes and loads a Scene. |
| Additive | Adds a Scene to the current open Scenes and loads it. |
| AdditiveWithoutLoading | Adds a Scene to the current open Scenes without loading it. It will show up as 'unloaded' in the Hierarchy Window. |