Short answer: Custom ISO support lets you boot a VPS from your own installation image, so you can install an operating system the provider does not offer, such as pfSense, a BSD or Windows with your own licence. It requires KVM and a VNC or web console.
Custom ISO explained
You upload an ISO or give the provider a URL, attach it as a virtual CD-ROM, reboot and run the installer through the console. Container VPS (OpenVZ, LXC) cannot boot ISOs.
See it in the data
Frequently asked questions
Can I install Windows from a custom ISO?
Technically yes on KVM, but you must bring a valid licence and check the provider allows it.