A "Return To Service" is apparently something like a christening for a newcon boat -- the sub still has all of her sea trials and certs to do before she can start doing actual patrols or deployments:
The Michigan and its 154-member crew -- it will eventually have two crews operating on rotating schedules -- now begin certification and testing that can last more than a year before it will be available for deployment, said Lt. Kyle Raines, a Navy spokesman.Still, it's probably nice for the crew to see the light at the end of the shipyard tunnel.