Scene reload on entering Play mode is on by default. This means that when you enter Play mode, Unity destroys all existing sceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
See in Glossary GameObjectsThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary and reloads the scene from disk. As your project gets more complex, the time between pressing the Play button and the scene fully loading in the Editor increases.
When you turn scene reloading off, the process takes less time. Instead of reloading the scene from disk, Unity only resets the scene’s modified contents. Unity still calls the same event functions (such as OnEnable, OnDisable and OnDestroy) as if the scene were freshly loaded.
When you turn scene reload off, the time it takes to start your application in the Editor is no longer representative of the startup time in the built version. If you want to debug or profile exactly what happens during your project’s startup, you should enable scene reloading to more accurately represent the true loading time and processes that happen in the built version of your application.
Otherwise, turning scene reload off should have minimal effects on your project. However, because scene reloading is closely connected to domain reload, there are a few important differences:
private fields are not serialized as part of the regular build pipeline but are serialized as part of the Editor’s hot reloading of scripts. This is why private fields you modify in Play mode might reset to their original values on exiting Play mode even when scene and domain reload on enter Play mode are both off.private and internal fields of array and List type to an empty array or List object during domain reload and they stay non-null for runtime (non-Editor) scriptsA piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like. More info[ExecuteInEditMode] or [ExecuteAlways] don’t receive OnDestroy or Awake calls. While in Edit mode, these scripts might modify their own fields or those of other runtime scripts. To mitigate this, you can initialize any affected fields in an OnEnable callback with code inside a condition that checks the value of EditorApplication.isPlaying. For an example of this and more context on the importance of separating Play mode and Edit mode code, refer to the [ExecuteAlways] API description.For more details on the events skipped when scene reload is off, refer to Domain and scene reload execution order reference.