This feature is useful when you have a client that pays you a fix retainer every month and in exchange you provide a preset number of hours of service at no additional charges.  Hours exceeding these preset number 
of hours are billed at your regular hourly rates for the case (by the way, you can set different hourly rates of timekeepers for individual cases/matters).  If you don't provide the preset number of hours of service in a particular month, you still bill the minimum retainer to the client. 

To setup a case for this billing scenario -> Click on "Cases (Projects)", select a desired case. On the right side tabbed pane, click on "Other Info" tab. Create a field named "MONTHLY_RETAINER" and add the field value XXXX (amount of the retainer in numeric format, e.g., 1450.50 or 2500). Repeat the same process to create another field named "MONTHLY_RETAINER_HOURS" and enter number of hours of "at no additional charge" service for the given monthly retainer (that is, how many hours of service you have agreed to provide at no additional charge for the monthly retainer amount). 

IMPORTANT: You must create invoices for separate months, that is, do not mixed two months into one invoice. The retainer billing will only be reflected in PDF/Word Invoices.