The goal of Windows Virtual Desktop Access is to simplify licensing requirements in a virtual environment by licensing the devices that seek access to virtual desktops, instead of licensing the virtual desktops themselves.
Because VDA is included as a feature of Software Assurance (SA), primary users of devices covered by SA can access their virtual desktops at no extra charge. Microsoft defines a primary user as someone who has used the computing device for more than 50% of the time in a 90 day period.
If the user wishes to access a Microsoft VDI from a device that is not covered by Software Assurance, however, a separate Windows VDA license is required. Such devices include thin clients, zero clients and third-party devices such as contractor-owned PCs. As of this writing, a separate VDA license costs $100 per year, per device.