Windows Server 2008 R2 Enterprise Server License
Windows Server 2008 R2 Enterprise is an advanced server platform that provides more cost-effective and reliable support for mission-critical workloads. It offers innovative features for virtualization, power savings, and manageability and helps make it easier for mobile workers to access company resources.

Hyper-V
Windows Server Hyper-V, the next-generation hypervisor-based server virtualization technology, allows you to make the best use of your server hardware investments by consolidating multiple server roles as separate virtual machines (VMs) running on a single physical machine. With Hyper-V you can also efficiently run multiple different operating systems—Windows, Linux, and others—in parallel, on a single server, and fully leverage the power of x64 computing. Hyper-V provides a dynamic, reliable, and scalable virtualization platform combined with a single set of integrated management tools to manage both physical and virtual resources, enabling you to create an agile and dynamic data center.

Maintenance
Software Assurance (SA) is Microsoft's maintenance program that provides product support, software upgrades, and more.

Licensing
A Server License is required for each operating system environment running Office Communications Server 2007. A User/Device CAL is required when a user or device is directly or indirectly accessing a Windows server. Additionally, if the user or device is accessing or using the Remote Desktop Services functionality of Windows Server, a Remote Desktop Services CAL is also required. A minimum of 5 CALs is required.