The overdraft account is an account usually associated to a credit card with a credit line (allows negative balance).
Compared to a standard account, the overdraft account can have this overdraft limit associated to it, and therefore the available balalnce is calculated automatically. In case you have a credit card, following the debt is very important while you also need to see your financial resources available at any time.
When entering the account balance, the value needs to be the real account balance. For example, if you have a $5000 credit limit and:
you haven't spent or deposited anything (the ATM would report $5000 available), the correct balance is $0;
if you spent $1000 (the ATM shows $4000 available), the correct balance is -$1000;
if you deposited $500 (the ATM shows $5500 available), the correct balance is $500;