I think that the Agents of Atlas predates the Avengers, but they also are the backup team. They're the team that Black Widow and Hawkeye find themselves on. It's where Swordsman ends up. Maybe Deathlok or Quake. It's basically what the TV show Agents of SHIELD is about.
It's the lower decks of the Avengers / SHIELD. These are the people who first look into 0-8-4's. It's the first line of defense for things that look safe to begin with. It's the last line of defense for things that have become four alarm fires.