That's a representation of building cash prices and earnings. The system's set up in a following way:
1K = 1,000
1M = 1,000,000
1B = 1,000,000,000
1T = 1,000,000,000,000
1aa = 1,000,000,000,000,000
1ab = 1,000,000,000,000,000,000
1ac = etc.
Once you reach surpass 1az earnings, the counter moves up to 1ba, and follows the same pattern as before (1bb, 1bc, 1bd, etc.) .