Scenes are instructions detailing how various HomeKit-enabled accessories should function at a particular time.
For example, you could construct a scene that tells the accessories in your home to adjust themselves in various ways, all at the same time. Lights could turn off, thermostats could change the temperature, and the like, all by selecting a certain scene.
HomeKit only supports 50 or less actions in a scene.
Apple includes four pre-defined scene templates that can be customized based on a certain scenario.
I get the error “Object already exists” when adding an action to a scene
Try to restart your device and try again. If the issue persists, you have to delete the scene and create a new one because of a HomeKit bug in iOS 9. If one of the built-in scenes is affected, you have to reset your HomeKit configuration in iOS Settings > Privacy > HomeKit.