Sometimes, I think the number refers to the cell count: A2, C2, C3, E1, E2, G2, M2, M6, U2, Z2. Other times, it refers to the voltage: 3P, 6P, 9P. Of course, in the first group, the letter precedes the number, and the second group is the reverse of that. I can't figure out a few of their names, like the 10X, for example.