Each occurrence of a retainer is called an instance. If you had a monthly retainer for the previous year, then you would see twelve instances of your retainer. When time logs or expenses are logged against a retainer, then they will be applied to a particular instance of that retainer, as determined the date on which the time or expense log was created.
When viewing a retainer you will see information about the current instance of the retainer. This makes it easy to see how your currently tracking against the retainer.