VUM service is a tool available in vSphere 6.5 that allows you to easily manage patches and updates for VM, hosts, and vApps installed in the virtual environment. As compared to previous versions, VUM no longer requires the installation of an additional external Windows server because in vSphere 6.5, the Update Manager server and client components are now part of the vCSA.
VUM is installed during the vCSA installation procedure and it's enabled by default. VUM uses a PostgreSQL database that is bundled with the appliance to store its data. Although both vCenter Server and VUM share the same PostgreSQL database, they use a different database instance.
In vSphere 6.5, a Windows based Update Manager 6.5 instance cannot be connected to a vCSA 6.5 during the installation procedure because it will fail with an error.
If the Update Manager doesn't have access to the internet, you can install the optional module Update Manager Download Service (UMDS) to download virtual appliance upgrades, patch binaries, patch metadata, and notifications. UMDS must be installed on a machine with internet access and, in version 6.5, it's available for both Windows and Linux based OS.