Retail distribution of Windows 8 has since been discontinued in favor of Windows 8.1. Unlike 8, 8.1 is available as "full version software" at both retail and online for download that does not require a previous version of Windows in order to be installed. Pricing for these new copies remain identical. With the retail release returning to full version software for Windows 8.1, the "Personal Use License" exemption was removed from the OEM SKU, meaning that end users building their own PCs for personal use must use the full retail version in order to satisfy the Windows 8.1 licensing requirements. Windows 8.1 with Bing is a special OEM-specific SKU of Windows 8.1 subsidized by Microsoft's Bing search engine.
What is the primary difference in Windows 9.1? 