This is a measure of the amount of BC in your treasury. All expenses are covered from it. It does not take into account income or any other economic ranking. If your treasury drops below -500 BC, ALL production in the empire is halted until the treasury goes over -500 BC. In low-difficulty games (beginner and lower) is any debt over -500 BC automatically converted into -500 BC at the start of the next turn.
There are two undocumented penalties that are triggered by certain amount of BCs in treasury:
- When treasury increases over 10,000BC the trade revenue is decreased by 50%
- When treasury increases over 20,000BC the tax revenue decreases by about 20%
Additionally, there's a cumulative penalty to morale for a long-time negative treasury. A popup will alert the player when their treasury has been in the red for a long time, which does not happen for the other two penalties above.