The Archon-class cruiser was a type of starship in service beginning shortly after the founding of the United Federation of Planets in 2161. Similar in appearance to the Daedalus-class ships of the Earth Starfleet that were later integrated into the Federation Starfleet, the Archon class was larger.
The prototype of this class was constructed from the spaceframe of what would have been the Horizon-class USS Gauss. Archon class vessels have a different internal configuration and greater endurance than Horizon-class ships.
USS Republic was a Constitution-class structural test ship made from an Archon-class spaceframe, hence the low registration number in comparison with most other vessels of the Constitution class. (USS Enterprise Heavy Cruiser Evolution Plans, Ships of the Star Fleet)