Both the Agent-Reservation and the Agent-Discharge are functions to prevent hunts from multiple UCC's at the same time. This may happen if the same agent is registered at multiple UCC's. Before introducing the Agent Service, UCC's did not share any information. Therefore, UCC's were not able to detect if a particular Agent was involved in a Hunt. Resulting in two calls at the same time to one agent.
The Agent Service shares reservation and discharge information among all UCC's. If an Agent is reserved, it reserved for any UCC. The same counts for Agents in discharge. If an Agent is in discharge, it is in discharge for any UCC. These functions are only active if configured.