Would have been?

Jaime closed his eyes and let the wine and Qyburn do their work. “Tell me of the battle.” As keeper of Harrenhal’s ravens, Qyburn would have been the first to hear the news. (Game of Thrones)

Context: In this story people used ravens to send messages and Qyburn kept the ravens.

Hi. I think I can roughly understand this "would have been" but I'd like to know how you call this usage.

Thank you.