Your Constitution Class vessel has been mothballed, and in return you receive a vessel of your choice from the 24th Century Starfleet flotilla. This includes models such as the Galaxy Class, the Sovereign Class, the Intrepid Class, the Defiant Class, or even the refitted Excelsior Class. These are just examples, and if you can think of a Federation ship from the era that you'd like, feel free to take it.