There are various types of retainer billing and accordingly various methods of handling retainer billing.

  Using Tabs3 Trust Accounting (Trust) for Refundable Retainer Billing
Tabs3 Trust Accounting (Trust) can be used to handle retainers--particularly refundable retainers.

  Using Progress Billing to Handle Retainers
The Progress Billing feature can be used to handle retainers for some situations.

  Using Tabs3 Billing Client Funds to Handle Retainers
Tabs3 Billingallows you to bill for retainers and keeps track of retainer activity. This feature is designed to be used for non-refundable retainers. 

  Entering Payments to Handle Retainers
You can simply enter retainers as payments for clients if you do not want to send a bill for the retainer.

See also:   Retainer Billing Examples
  Comparing Different Methods of Handling Retainer Billing
  Methods of Billing