If you’ve been running a lot of Windows Server-based workloads on a hypervisor, you’re probably familiar with Microsoft’s licensing policies for any virtualization tasks that you perform. Specifically, if you’re purchased Windows Server Data Center edition for a server, you have rights to run unlimited server “OSEs” on that server. An OSE is an operating system instance. If you wanted, you could run 100 copies of Windows Server Enterprise on that host and be within licensing compliance.
It’s important to understand that this licensing right extends only to server editions. You cannot run a desktop version of Windows under this license. If you add a virtual machine running a desktop version of Windows, that copy of Windows must be licensed separately. The desktop workload cannot be run under the server’s unlimited virtualization rights license.