This topic is broadcast from the ISceneLoadingService when a scene of a Chunk begins loading. The ValueMessage<string> carries the identifier of the scene that is about to load. Since ISceneLoadingService is implemented by each individual game engine differently, the nature of the identifier is highly dependent on the implementation details of the engine. The Unity module provided for this architecture uses the scene's address as identifier, since it's dealing with Addressable scenes. Other implementors might use different identifiers. This is an Async Topic whose callbacks can be long running Tasks. The service will await for all listeners to complete executing their callbacks before resuming the operation.