A. Any device accessing a virtualized Windows client OS needs virtual desktop access rights. PCs that are covered by Software Assurance include the virtual desktop access rights. If consultants are using their own machines (or machines that aren't covered by SA), each device requires Windows Virtual Desktop Access (VDA), which is an annual, per-device subscription that costs $100.

Note that, as with most other Microsoft licenses, you have transfer rights for every 90 days. So you could purchase a pool of Windows VDA licenses that would cover the number of devices that are connected over a 90 day period then transfer the licenses between consultants as they come and go.

For all the details on VDI licensing, see Microsoft's FAQ on the topic (pdf).