The Angel City Elite (ACEs) is an independent band of mercenaries on the Frontier, a ragtag group of soldiers, freelancers and Pilots that are under the command of Robert "Barker" Taube. They are presumably based out of the Harbor District of Angel City, the metropolis in which Barker was living and had garnered sympathy from before his abduction in the Titanfall mission Get Barker.
Though they are an independent mercenary group, Barker and his Elites are strongly sympathetic to the cause of the Frontier Militia and are regularly under contract with them. Since the events of the Battle of Typhon, the ACEs now have a more personal interest in "[seeking] answers to other mysteries within the Frontier."
The Angel City Elite is one of the playable factions in the multiplayer of Titanfall 2, with Barker as its field commander.