Oracle Receivables :On-Account Vs Unapplied Receipts
On-Account and Unapplied are almost same with some differences. Both won't change Customer Aging (Outstanding).
Unapplied means Receipt is available for application to Transactions.
On Account receipts would be if we are not sure which transaction/debit/credit memo you would like to apply this receipt to you can just put on account to reduce the customers balance and to give a true picture of what the customer owes you .
You then can go back into that receipt, go to the Applications form and Unapply the ON Account application and then you can select from the LOV any transaction/debit memo or credit memo you decide.