So the distinction is between being able to specify either "This is the highest priority handler for this" or "This is the lowest priority handler for this". For implementing the framework, it's useful to be able to specify the default or lowest priority handler, so the greater the number the greater the priority makes sense. For using the framework, it's probably more useful to be able to specify the absolute highest priority handler, because most users will have only 1 handler per message type and they won't want it to be overridden.