Account balance refers to the difference between the total amount debited and the total amount credited to an account.
To check your customer's payment balances go to the customer dashboard, in Relationships -> Customers
You can also go into a customer's account and view their balance, past due amount and credit limit:
If you want your customers to see the outstanding balance on their account or their 30 day spend, you can do that in the Configurations -> B2B Portal Settings.
Customers can see their account balance in the B2B Portal by going to My Account -> Account Info -> click on the account name.