Windows 10 has two core editions: Professional and Enterprise (see editions comparison). It is licensed on a per device basis. One license grants the right to install and use one instance of the product on one device in a physical or virtual operating environment.

Windows Desktop Offerings Available Through Volume Licensing

If no operating system is installed on your organization's computers and you want to install Windows 10 Pro, then you need to purchase one of the following:

• MICROSOFT GET GENUINE KIT FOR WINDOWS 10 PRO - LICENSE - 1 PC - Legalization - Open License - Single Language
• WINDOWS 10 PRO - LICENSE - 1 license - download - ESD - 32/64-bit - All Languages

Below is an overview of the different types of Windows OS piracy:

Mislicensed Software

• Purchased PCs without an OS pre-installed or with Linux, FreeDOS  or OOS (other operating system) installed, and then used a  Microsoft VL Agreement to install the full Windows desktop operating system.

Version Piracy

• Purchased PCs with Windows Home Premium, Home Basic, or Starter editions and then used a VL Agreement to upgrade to Windows Business or Windows Ultimate. 
• Purchased PCs with Windows Home edition or Starter edition and then used a commercial VL Agreement to upgrade to Windows Professional.

Pirated Software

• Installed Windows software using leaked or stolen Volume License Keys (VLKs).
• Downloaded software online or from unknown or counterfeit sources.
• Attempted to save costs by doing in-place motherboard upgrades without acquiring new Windows licenses on those PCs.
• Purchased from an OEM that used a single license on multiple PCs (called hard disk loading). 

Windows Legalization Solution for scenarios written above: 

MICROSOFT GET GENUINE KIT FOR WINDOWS 10 PRO - LICENSE - 1 PC - Legalization - Open License - Single Language

Licensing Logic: What's the difference between OEM, Retail and Volume Licenses?

There are several different channels through which you can purchase a license. These are FPP (Full Packaged Product), OEM (Original Equipment Manufacturer) and Volume Licensing. So, what are the key differences between the channels and what impact do they have on your licensing rights? The main difference between the channels is the usage rights you receive with the software.

Let us start off by looking at FPP licences (Retail).
FPP licenses are purchased at retail and provide a quick and convenient way for customers requiring less than five licenses to purchase their software. Licenses sold through FPP are either full licenses or upgrade licenses. A full license does not require any pre-existing versions of the software to be on the machine it is installed on, whereas an upgrade license allows you to cost effectively upgrade to a newer version of software that you are already licensed for.

When you purchase FPP software you are entering into an agreement with Microsoft stating that you will adhere to the usage rights associated with the software. These rights include details such as downgrade rights, which you generally do not get with FPP software; how many devices you are allowed to install the software on; whether you have the right to move the software to another computer (transfer rights) or not; and nowadays whether you are allowed to use the software for commercial use or not.

The usage rights for FPP software are laid out in the Microsoft Software License Terms document, which can be downloaded from this site.

Microsoft also provides limited support services for software that has been licensed via the FPP channel, details of which can be found here.

OEM software is software that comes pre-installed when you purchase a new computer. So, for instance, when you purchase a new PC it might come with an OEM licensed copy of Windows 10 Pro pre-installed on it. One of the key benefits of this is that you then know that the software has been installed correctly and is working properly and if there are any issues it is the responsibility of the computer manufacturer or installer to provide support rather than Microsoft.

Again, OEM software usage is ruled by the Microsoft Software License Terms document. So, for instance, it will detail that with Windows 10 Pro licensed via OEM you do get limited downgrade rights in that you can downgrade as far back as two earlier versions if you so desire, either to Windows 8 Professional or Windows 7 Professional. But that there are no transfer rights, so the software may not be removed from the PC it was originally installed on and installed on another, however, you are allowed to transfer the PC complete with the OEM licensed software to a new user if required.

Officially only computer builders are allowed to buy and install OEM licenses on computers for sale to someone else. A computer builder cannot use an OEM license on a computer for their own use.
Officially, consumers/end users are not allowed to use OEM licenses on their own computers.

You can find Windows 10 Pro OEM license on AMAZON for approximately $99, but consumers/end users are not allowed to use OEM licenses on their own computers.

The third channel you may purchase software through is Volume Licensing. There are a number of volume licensing agreements you can purchase through and these agreements have been tailored to suit the differing requirements of all sizes and types of organization. There are agreements aimed at the small to medium sized business in the form of Open and Open Value, and agreements aimed at medium to large organizations in the form of the Select Plus and Enterprise Agreements.

Usage of software purchased through these agreements is controlled by several factors; the agreement itself may contain some usage rights specific to purchasing through that agreement, but the usage rights per product can be found in the Product List document and the Product Use Rights (PUR) document. You can download the Product List and PUR here.

For example, with an Open License Agreement the transfer rights for software are detailed in the Open License Agreement document itself, stating that licenses may only be transferred once they have been fully paid for and that they can only be transferred to affiliates or a third party as part of a divestiture or merger. So, it is always important to check the agreement documents as they will detail rights that are specific to the agreement itself, but what about the more general terms?

Well this is where documents such as the Product List and PUR come into play. The Product List provides monthly information about Microsoft Software and Online Services licensed through the volume licensing agreements, and details software availability through the different agreements, as well as Software Assurance migration paths, step-up licenses and available promotions. The PUR goes into detail on the general volume licensing terms for each licensing model plus any product-specific use rights. All these documents together can be used to determine the license terms for products purchased through specific agreements.

For example, when you purchase Windows 10 Pro through Volume Licensing the license is an upgrade license only. This means that the device must already be licensed for a qualifying operating system before installation of the Windows Upgrade. The Product List contains all the detail behind this including a list of the qualifying operating systems per agreement, whereas the PUR goes into more detail on where Windows 10 Pro may be installed and who can access the software either locally or remotely.

Other important product rights you receive when purchasing your licenses through volume licensing are re-imaging and full downgrade rights. Downgrade rights are detailed in the PUR, and you may find detail on re-imaging rights in the specific agreement documents. However, there is a very useful re-imaging brief that goes into all the detail you may need and you can download that here.

So, to wrap up, we have seen that there are a number of different ways for you to purchase your licenses, and the availability of software does vary through those channels, but the main impact on the licenses of buying through FPP, OEM or Volume Licensing is the usage rights you gain with the software.