bank... You can save your money from the running season for the next season (you couldnt because you havent raced in 1991 season).
If i understood it right (first season we use this additional thing) you can have max 1000 credits cash at the start of a season. the rest of your money you saved comes into your bank. You can use it if you are in the secnd half of the championship table after half the seaso. or something like that
Also there is team money. With this money you and your teampartner can upgrade the car. The team money is a prize from last years championship position of the team.
With your own money you acn upgrade the car, too, of course