March 2020 cumulative update for Windows 10 version 1909, 1903, and version 1809 is March 2020. The update is rolling out to PCs with November 2019 Update, May 2019 Update and October 2018 Update. There are only security enhancements for the system, browsers, core components and other basic functions. Microsoft is aware of one bug: When using Windows Server containers with the March 2020 updates, you might encounter issues with 32-bit applications and processes. If you own multiple PCs or if you would like to patch the PCs manually, you can learn more about it here.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-cumulative-update-kb4540673-and-kb4538461-released/