StrikerS takes place after the epilogue of Magical Girl Lyrical Nanoha A's. While the epilogue connected A's to a world six years later, StrikerS advances the timeline another four years, where the main characters have gone on to work for the Time-Space Administration Bureau (TSAB) and come upon a new case. The series focuses more on team-based battles and bureaucracy rather than individual rivalry and school life, due to the change in character dynamics. The name StrikerS refers to an SS rank given to top mages, much like how A's refers to A rank mages. - Taken from Wikipedia