Question: In Online Credit-Control, what is the primary purpose of sending a 'Reserve Units Request' from the Charging Trigger Function (CTF) to the Online Charging Function (OCF), and how does unit determination impact this request?
Option 1: To immediately deduct units from the subscriber's account for a rendered service, with centralized unit determination always specifying the number of units.
Option 2: To reserve a number of units for a service, where in case of decentralized unit determination, the CTF specifies the service key, and the OCF determines the units.
Option 3: To reserve a number of units for a service to be provided by the CTF, with unit determination (centralized or decentralized) affecting how the request is formulated (either specifying a service key or the number of units).
Option 4: To refund previously reserved units, always using centralized unit determination.
Explanation: The correct answer accurately reflects the purpose and process of 'Reserve Units Request'. It highlights that the request is for reserving units for a forthcoming service, acknowledging the dual approach to unit determination (centralized and decentralized), crucial in telecom networks for effective charging management.