This update enables you to use Microsoft SQL Server 2017 Express edition as a witness role in a database mirroring session.
This update is included in Cumulative Update 1 for SQL Server 2017.
About SQL Server 2017 builds
Each new build for SQL Server 2017 contains all the hotfixes and security fixes that were in the previous build. We recommend that you install the latest build for SQL Server 2017.
Learn about the terminology Microsoft uses to describe software updates.