If you choose to have VMware ESXi installed on your Mac server, you'll have access to a powerful management layer for virtualizing on Mac hardware. VMware ESXi is a hypervisor for deploying and serving virtual machines running many different types of operating system. Utilizing Mac hardware is the only way to legally virtualize macOS1.
In the past, it was only possible to manage ESXi from the command line or via a desktop client installed on Windows. In early 2016, VMware began including an embedded HTML5-based VMware Host Client that allows users to connect to and manage a single server running ESXi for the management of host resources and virtual machines. There is no extra cost to use this tool; it's included in all new installs of the latest version of VMware on Mac servers at MacStadium.
1. macOS may be interchangeable with OS X, the name used for previous versions of Apple's Mac operating system. ↩