Probably many people know that Japanese limited express trains have reserved cars, non-reserved cars and Green cars. Green cars are first class and seat must be reserved for limited express trains. Then, you can sit down anywhere on non-reserved seat if the seat is not occupied. Then, if you do not have seat reservation ticket, you can not sit down on vacant seat on reserved cars.
Actually limited express tickets are also divided into three types. First one is Limited express Green ticket, Limited express reserved seat ticket and Limited express non-reserved seat ticket. These tickets are for Green cars, reserved cars and non-reserved cars.
However, JR East has introduced a unique seat reservation system which is little bit confusing. It is called limit express unreserved seat ticket. Limited express trains on Chuo main line, Joban line and Takasaki line now have only reserved cars and Green cars. Which means there is no Limited express non-reserved seat ticket.
On the other hand, limit express unreserved seat ticket is possible to sit down on vacant seat on reserved cars. If the seat is vacant, red light of seat reservation status above your seat is on. If it is red you do not have to ask the passenger who is sitting next to the vacant seat if the seat is taken or not. If the light is green, it means that seat is reserved. Someone is sitting or went to the lavatory and he or she will back soon. Then, there is also a yellow light, which means that the seat will be reserved seat soon.
The status will change to yellow from red before arriving at the next station. For example, when you get board on Azusa train at Matsumoto station, and red light is on, you can sit down. However, the status might change to yellow shortly before arriving at Kofu station later. In this case, you have to give up your seat and find a seat with status = RED.