A subtreasury is a place where money is kept, like a bank.
A subtreasury is a place where money is kept safe and is usually run by the government.
Examples:
The subtreasury holds a lot of money.
I will go to the subtreasury to get some money.
The subtreasury is closed on weekends.
The subtreasury is like a big piggy bank.
The subtreasury is so full of money, it's like a treasure chest!
The subtreasury is where the government keeps its money safe.