RFC 4006 defines both Event Charging with Unit Reservation (ECUR) and Session Charging with Unit Reservation (SCUR). Both charging events are session-based, and require multiple transactions between the CTF and OCF. ECUR begins with an interrogation to reserve units before delivering services, followed by an additional interrogation to report the actual used units to the OCF upon service termination. With SCUR, it is also possible to include one or more intermediate interrogations for the CTF in order to report currently-used units, and to reserve additional units if required. In both cases, the session state is maintained in both the CTF and OCF.

For both ECUR and SCUR, the online charging client implements the "CLIENT, SESSION BASED" state machine described in RFC 4006.

For direct debiting, Immediate Event Charging (IEC) is used. With IEC, a single transaction is created where the OCF deducts a specific amount from the user's account immediately after completing the credit authorization. After receiving the authorization, the CTF delivers services. This form of credit authorization is a one-time event in which no session state is maintained.

With IEC, the online charging client implements the "CLIENT, EVENT BASED" state machine described in IETF RFC 4006.

Unit determination refers to calculating the number of non-monetary units (service units, time, events) that can be assigned prior to delivering services. Unit rating refers to determining a price based on the non-monetary units calculated by the unit determination function.

It is possible for either the OCF or the CTF to handle unit determination and unit rating. The decision lies with the client application, which controls the selection of AVPs in the credit control request sent to the OCF.