'Minimal negative interest rate'? in 'Saving account'

What is minimal negative interest rate? Or what is maximal negative interest rate? It is not defined in task??? Excercise is called ‘Savings account’, task number 1 and test number 10!!!

I can’t really speak about why “minimal” and “maximal” words were chosen. I think it’s meant to be read as “test the interest rate for minimal negative balance”.

The instructions tell you what the interest rate should be when the account balance is less than zero. That’s the value this test expects.