The Current Loan system in Simcountry, is based off of the Reserve Banking system in the real world.
In the real world:
When large institutions have more cash than they need, they deposit it in the Reserve Bank. Utilising what is called the "deposit facility".
When financial institutions have less cash than they need, they borrow money from the Reserve Bank.
Usually there are more borrowers, than depositors. So the Reserve bank makes up the difference by printing money, increasing the money supply.
If there is to much money in the system. The Reserve Bank raises interest rates, to encourage more institutions to use the deposit facility, and fewer to borrow money. This reduces the Money Supply. Helping to control inflation and regulate the economy.
In Simcountry:
The Reserve Bank is represented by a "World Bank" (really a Reserve Bank) controlled by the GM's. When there are enough depositors (countries/enterprises willing to lend money), to cover requests from borrowers. The interest rate the game uses is dropped to 12% per real year, or 0.06575% per game year, for those transactions.
When there isn't enough money to cover borrowing requests. The "World Bank" (Reserve Bank), makes up the difference, at 15% per real year, or 0.08219% per game year. This helps to stabilise the money supply, by raising the interest rate paid, when there are to many people trying to spend more money than they have.
Follow up notes:
While 0.06%-0.08% per game year may seem like an extremely low rate. This rate is actually higher than the current rates, set but Reserve Banks in the real world, in the European Union, Japan, Switzerland, Denmark, and Sweden. Additionally even the US Federal Reserve Bank, had an interest rate equivalent to the levels in Simcountry, between 2009 and 2016. So the Simcoutry levels are actually pretty realistic for the transactions they are simulating.
Signed President of DanNation on LU