Table of Contents
What is Proxmox?
Proxmox has three main branches,
Proxmox Virtual Environment
Proxmox VE is a complete open-source platform for enterprise virtualization. With the built-in web interface you can easily manage VMs and containers, software-defined storage and networking, high-availability clustering, and multiple out-of-the-box tools on a single solution. 
Proxmox Backup Server
Proxmox Backup Server is an enterprise backup solution for backing up and restoring VMs, containers, and physical hosts. The open-source solution supports incremental backups, deduplication, Zstandard compression, and authenticated encryption. 
Proxmox Mail Gateway
Proxmox Mail Gateway is an open-source email security solution, protecting your mail server against all email threats from the moment they emerge. The full-featured mail proxy can be easily deployed between the firewall and your internal mail server in just a few minutes. 
Proxmox is open source, but does offer an enterprise and a Support Subscription option.  But this is not needed to use the product. From the Proxmox support subscription page.
“Proxmox Virtual Environment's source code is published under the free software license GNU AGPL, v3 and thus is freely available for download, use and share. You can download the Proxmox VE ISO installer or inspect the code in the public code repository (git). A Proxmox VE Subscription is an additional service program designed to help IT professionals and businesses to keep your Proxmox VE deployments up-to-date. A subscription provides access to the stable Proxmox VE Enterprise Repository delivering reliable software updates and security enhancements, and to technical help and support. Choose the subscription that's best for you. Our four plans are flexible and scalable to your business needs. The subscription model is based on the number of physical servers and CPU sockets. The subscription period is one year from purchase time, and gives you access to the broad infrastructure of enterprise-class software and services.”