FreeRADIUS Authorization Question

Hi List Greetings How do we authorize users using FreeRADIUS such as way that ClientA can only use VOIP service, ClientB mail only, ClientC internet only and so on ? Do we do by using some attributes ? or possible in unlang only ? i do not have logical diagram in my mind, how would i achieve that, using port numbers of services ? Could someone help me to understand ? --THKS --RM