The game currency system is set up like this:

1 K = 1,000 (one thousand)
1 M = 1,000,000 (one million)
1 B = 1,000,000,000 (one billion)
1 T = 1,000,000,000,000 (one thrillion)
1 aa = 1,000,000,000,000,000 (one quadrillion)
1 ab = 1,000,000,000,000,000,000 (one quintillion)
1 ac = etc.

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