Introduction SharePoint 2013 introduces a new model on how Site Collections are upgraded using the new Deferred Site Collection Upgrade. To keep it simple it means that SharePoint 2013 can run a Site Collection in either 2010 mode or 2013 mode, and SharePoint 2013 contains a lot of the SharePoint 2010 artifacts (JS files, Features, Site Definitions) to handle this. When you’re doing a content database attach to upgrade from 2010 to 2013, only the database schema is upgraded and not the actual sites (by default).