The Amount unposted field is used for amounts in a payment plan outside of the very next billing cycle. Playground only shows parents the balance due on the next billing cycle instead of the entire payment plan amount.
In the example given above, Peter Li's tuition rate is $500/month for the months of September 2021 through August 2022. Peter's guardians have paid tuition from September through May, this is shown by the $4,500 Amount paid field. Peter's guardians did not pay tuition for June, which is shown with the $500 Amount overdue field. Peter's guardians owe $500 for July tuition. The combination of June's overdue amount and July's current balance ($1000) is due on the next billing due date of July 1st. There is one payment left in the payment plan that is not due on the very next billing cycle, this is shown in the $500 Amount unposted field.