It is same as previous extension that any agent can register on some soft phone. However, because of the introduction of call center related features, some modification has also been made to this type of the extension. The new features of this extension type (Agent / Device) are given below:
Agents can subscribe call queues now. That means when an agent subscribes to a call queue he/she can receive incoming calls in that queue.
Agent can login/logout by dialing something from his phone. So he can start and stop receiving incoming calls. If agent is logged in, he/she can receive incoming calls and if logged out, stops receiving incoming calls.
These features allow to choose which extensions can log in to and take calls from the queue. You can specify whether calls are sent to available agents in order or randomly.
For more info Set up Call Queue in Autodialer