I do not know much about Renesas Microcontrollers. By definition I would say that an Asynchronous microcontroller does not require and external clock whereas a synchronous one would.
Given that microcontrollers generally have a specific function, I imagine that the difference lies in what system it will be working with.
The most important thing for the difference is that the assyn. communication data rate is predetermined and making transmitter and reciever nodes more complex