Microsoft Windows Licensing Model
Windows Upgrade License requirements
Different product families require different licensing models. For example, a desktop PC program, such as any Microsoft Office suite, requires a license for each device that is running the program. Customers can install any number of copies and any prior version on the device or on a network device.
Desktops
A desktop PC operating system, such as the Windows operating system, requires a license for each device accessing the licensed product (locally and remotely). Note: Only one copy can be installed on the device.
In Volume Licensing, the desktop PC operating system license is an “upgrade license.” You can only acquire upgrade licenses for devices for which you have already licensed a “qualifying operating system,” either preinstalled on a PC through a PC manufacturer (OEM) or as full-package product (FPP) from retail. Information about "qualifying operating systems" is available on the Volume Licensing for Microsoft products and Online Services web site.
Servers
Meanwhile, most server products require a license for each running instance of the server software you run on a server--whether in a physical or virtual operating system environment (OSE). However, some products provide broader use rights. For example, a Windows Server Standard edition operating system license permits a running instance in up to two virtual operating system environments, plus one host instance on the physical device at a time on the licensed server. With some products, such as Windows Server Datacenter edition, any number of instances can run simultaneously. Both Windows Server Standard and Datacenter editions require that each physical processor be licensed on the licensed server.
Server operating system products require a Microsoft Client Access License (CAL) for each user or device that accesses the server software. CAL's are version specific and must be the same version or later than the server software being accessed. CAL's permit access to servers licensed by the same entity. They do not permit access to another entity's licensed servers.