Where it is
It is the last row of seats in a car. The space between that row and the wall behind it is reserved for the large bags of those passengers.
Who needs it
Anyone with a bag over 160 cm in total. Travellers with smaller bags can still book it if they want their bag close by.
Extra large baggage compartments
Some trains also offer lockable or reservable baggage spaces near the doors. If they are offered when you book, they work as an alternative to the last row seat.
Travelling with a Japan Rail Pass? Seats with an oversized baggage area are included at no extra cost. Get your Japan Rail Pass