After your payment application has been billed out 100% and there is only retainage remaining, you can send a request to the project manager to release the payment application's retainage.
To release retainage, view the final payment application that is available to submit by navigating to Projects > Your Project > Documents tab > Pay Apps.
Once you are in the payment application, there is a button located in the upper right corner called Request Release of Retainage. After you click the request release of retainage action, the payment application will be assigned to the first person in the approval chain. The first approver, can then release the retainage and approve the final payment application.