You can execute the tasks with or without the UCallback parameter and this will compile and even run perfectly, but it doesn't actually do something. It also doesn't log it doesn't do anything, which would have been nice, since not providing it is allowed by the interface. If it is allowed but not supported in that specific task, it should at least log that so that users are aware of it. If it is not allowed, do not foresee this in the interface then and make the callback parameter obligatory. And if not forced/reported in the code, then it might be good to explicitly mention this in the documentation that some tasks do or not do stuff depending on whether or not there is a UCallback provided.