Windows Update categorizes software updates based on the update's importance. There are three categories for updates: important, recommended, and optional.
Important updates. Important updates help keep your computer more secure and reliable, protecting your computer and your privacy. These updates include security and critical updates, as well as reliability improvements.
Recommended updates. Recommended updates help keep your software up to date and your computer running at its best. If you selected Use recommended settings when you set up Windows Update, then recommended updates will be shown together with important updates. If you selected Install important updates only, recommended updates will be shown together with optional updates. Recommended updates include software updates and new or improved features.
Optional updates. Optional updates include updates and software that you can install manually, such as new or trial Microsoft software or optional device drivers from Microsoft partners.