The USS Enterprise (NCC-1701-A) was a Starfleet Constitution-class starship under construction at Yorktown Station following the destruction of her predecessor in the Kelvin timeline. (Star Trek Beyond)
During its systems shakedown cruise to New Vulcan, a momentary upset in the ship's intermix ratio caused the Enterprise-A to appear in another dimension where it met up with the USS Ambassador, which was under command of Prince William of the Scarlett Army.
With the assistance of the Ambassador's crew, the Enterprise-A returned to its own dimension, where five other Starfleet ships--the USS Enterprise, the USS Enterprise-D, the USS Defiant, the USS Voyager and the Enterprise (NX-01).
Together, the seven crews helped save their universe from the Army of Inter-universal Chaos. (Golden Voyagers)