https://www.bleepingcomputer.com/news/microsoft/microsoft-to-retire-windows-10-delta-updates-in-2019/

The cumulative updates for Windows 10 are released every month with both security and non-security improvements in just one package. Microsoft uses three different types of updates, which are either full updates, express updates, or delta updates to make it easier for IT admins to stay up to date and secure.

Full updates have all the necessary components that may have changed since the last update. On the other hand, the express update would generate "differential downloads for every component in the full update based on several historical bases", but a user only has to download what files have been specifically changed. Delta updates are larger in size compared to express updates because they contain the full component, rather than just the changed files. They also require the previous month's updates to be installed.