Making Your SharePoint 2010 Customizations Visual Upgrade Aware

Much of our UI is aware of Visual Upgrade in that it conditionally renders based on the Site’s UI Version (2007 UI or 2010 UI). But SharePoint is built as a platform, so there are several ways in which your customizations can also be aware of Visual Upgrade.

https://blogs.msdn.com/sharepoint/archive/2010/04/14/making-your-sharepoint-2010-customizations-visual-upgrade-aware.aspx