You mean like: “Classic,” “Adventure,” and “Modern/boost/meta?” Classic would be 91-98, concluding with Sonic Adventure releasing in Japan. Adventure started in 98 and either concluded with 06, or some people will subdivide and say it ended with Heroes in 2003, so from Shadow until Unleashed, 2005-2008, are the “Dark ages.” Sega’s “Modern era” according to Generations was Sonic 06 to Generations. Because of the name, it’d be hard to say we’re not in the “modern” era at any point lol. For fans, “Boost” is every game from Unleashed until Forces, though I’m not sure if they include Lost World since it wasn’t a boost game. “Meta” refers to the writing style of Pontaff and Griff which debuted in Colors in 2010. This continued through to Forces. It’s a mess. I personally think of them as Classic, Adventure, and Boost though.