Boxes are a Japanese-style seating arrangement. Depending on which city the event is in, there are boxes for 1, 2, 3, 4, 5 or 6 people. You get an area of the floor and a cushion to sit on (it is elevated, so visibility isn’t a problem). This is the most ‘exotic’, but also the most expensive, way to see sumo. Box ‘A’ are the closest to the ring, then ‘B’, then ‘C’.
Please note that many tourists, especially if they are tall/wide or have back/knee problems, are uncomfortable in Boxes. If you think this might be a problem for you, you should consider buying Arena seats or a bigger Box. The 2-person Box ‘C’ in Nagoya and Fukuoka are full-sized Boxes designed for 4 people, but are sold for only 2 people, so you get extra space. Here is a photo of a Box in the Tokyo venue.
Arena seats are chairs like Western people would be used to from an arena in their country. In Tokyo, the seats are cushioned, more like what you’d expect in a movie theatre. Arena ‘A’ are the closest to the ring, then ‘B’, then ‘C’. Also, Arena ‘A’ have more cushioning than ‘B’, which have more than ‘C’. The Arena seats in Osaka, Nagoya and Fukuoka are hard plastic.