The in-game currency system is set up like this:

K = 1,000
M = 1,000,000
B = 1,000,000,000
T = 1,000,000,000,000
aa = 1,000,000,000,000,000
ab = 1,000,000,000,000,000,000
ac = etc.

We use this system to make big numbers easily recognizable and comparable.