An administrator can set a billing rate for each agent, customer or ticket.
You can select which roles are allowed to participate in billing.
Once you have selected which WordPress roles can participate in billing a customer, you can navigate to the WordPress user profile for the agent and enter their billing rate.
To set a billing rate for a client simply go to the user profile for that client and set the billing rate in the Client Hourly Billing Rate field. This rate will take precedence over the agent rate. So any tickets with time recorded will use the billing rate for clients if one is present in the client profile.
You can set a rate override for each ticket. There is a billing rate field in the TIME TRACKING metabox. Any rate entered there will override the rate specified in the client or agent profile.