Trains running in metropolitan area which are non-reserved cars have designated seats called priority seats for elderly, injured, physical handicapped, pregnant and other passengers who need special care. By the way, priority seats are commonly known as SILVER SEATS too.
This is same as in other nations marked with signage on the window or on the wall but usually the color of the seats are different than the others. Also you can see the signage from the outside of the car,
Please keep in mind that Japanese do not offer their seats when they are not sitting on priority seats, then those people who need the seats also do not expect to be offered the seats either. These people will find the priority seats by themselves and if they wanted to be seated, they will not come to the ordinary seats. Therefore if you see anyone who does not offer his/her ordinary seat to elderly person, please do not be upset to see his/her behavior.
Also sometimes it is rude to offer a seat to people who looks pregnant or elder. They might not be pregnant or old!