All the letter-number combinations indicate an exit that can be reached while underground. As you see it spans for a few blocks. Makes things convenient since it was raining outside, although I was in an exploratory mood anyway. One thing I haven't mentioned is most directions in Tokyo are given by stations. Like the sony building (see next image) is at exit B9. If you looked up directions somewhere, it would say you get to the sony building by going to the ginza station and exiting B9. A lot of places are given as distances from the nearest train station.
