B C Z
That just means that someone chose a race other than the first two. So if you want the fourth race combo you would put one coin on the first race, one coin on the second and one on the third. That's why you start the game with five coins -- in case you want the sixth race available.
Edit: Damn I type too slow.
Note that the 6th race is not actually available - it is just visible so people know what is coming next.