Piergiordgio Del Moro is probably the most powerful person in fashion after Anna Wintour. Some might argue he is more influential than Anna. He decides which girls have a career and which don't. He casts every big show and every big campaign and every big editorial.
Most people think that model agents have influence but none of them really do at the highest level. They are just admin. Pier controls everything. From who is cast in shows, to who appears on covers. He is the person who decides. The biggest models on earth need their agents to ask him for permission before they book jobs. They answer to him. Why? Because he is not going to invest time into a girl for a random agent to go and screw up all his work by booking them on some easy money job. All these careers are carefully strategised.
He is also very influential in photography, which a lot of people don't know. If he doens't like a team he will tell a magazine to change it. If they don't, then they don't have access to his 'stable' of girls. He can make or break a photographers career the same as he can with a model. He really is the gatekeeper to the inner circle. Sounds lame but it's the truth.
Brokaw has been second best casting director for a long time but still influential. There are a lot of big clients out there and PG can't do all of them. Brokaw does the rest. There are other good casting directors of course. Julia Lange, Larissa Gunn, Adam Hindle, Shelly Durkan etc to name a few. I'm missing a lot of them but there are 20 or 30 in the next tier down. None of them can really compete with the machine that DM (Pierre) casting is. If anything, they all copy and imitate.
It is just as political and polarised as any other power dynamic. PG girls will very seldom do Brokaw shows and vice versa. Sometimes they crossover, but not often. It's a sweet spot for a girl to be in the favour of both of them. Currently there's only 5 or 6 models worldwide that do both. Once you notice it you can't unsee it. If there's a Brokaw casted show that is 6 hours apart from a PG show you might see some of the girls cross boundaries. But as with this morning (max mara and Jill sander), not one single top girl will do a Brokaw show if it is in defiance of PG. Small clue but Aylah Peterson fell off for a reason. She did Brokaw stuff without permission.
That's the gist of it.