Television Show Numb3rs

What does a FBI Special Agent in charge do?

I've been googling this for like the last hour. I know that for most FBI offices there is a Special Agent in Charge that overseas that entire office. But in New York and Los Angeles the offices are so big they have an assistant director that does that but there are still Special Agents in Charge in those offices. My question is what do those agents do? Are they like in charge of a certain part of the FBI office?
I'm asking this because of a television show Numb3rs. I thought the main character was a SAC in the Los Angeles FBI, but at the end of the show he gets promoted to SAC and I was wondering what the characters future job would be.

SAIC is a promotion, much like a sergeant on your local police force. They perform the same duties, but also supervise the other agents and make decisions as necessary.

Numb3rs trailer- Cool Television Show

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