When you initially order a server with macOS1 on it, we don't do any operating system customization; it's macOS on the bare metal Mac as if you'd bought it at a store.
In the past we installed the VMware ESXi hypervisor and created a Linux virtual machine by request. Due to the complex nature of installing Linux and the many variations available, we now offer the base VMware ESXi installation with no virtual machines included. Customers may install virtual machines on their own.
One of the benefits of the hypervisor is the ability to install multiple operating systems on one system. If you're subscription is a Mac mini with quad core i7 CPU, you can run multiple virtual machines with various operating systems on the server and little in the way of stuttering or lag (subject to application use, of course).
1. macOS may be interchangeable with OS X, the name used for previous versions of Apple's Mac operating system. ↩