Retainers (also known as contracts) are ideal for managing recurring client work, maintenance quotas or periodic invoicing - such as licence fees.
A retainer is created against a company/client record, and work is tracked against the retainer through periods. You can configure your retainer to automatically create periods at certain intervals, and invoice up-front for a fixed amount or at the end of the period for the work done.
The retainers modules integrates seamlessly with the issues and projects modules so that you can even allocate work from those into a retainer period.
Sample view of periods against a retainer