This is a good documentation to advise on how Licensing Mode for Terminal Services 2008. Taken from Technet Article “Choosing the Licensing Mode“. These 2 paragraphs are very important. User CALs is still going by the honour system. That does not mean we blatantly violate MS Licensing requirement. But at least it gives us times to response in case we run out of licenses.
Which CAL you choose depends on how you plan to use the terminal server. When the Per Device licensing mode is used, and a client computer or device connects to a terminal server for the first time, the client computer or device is issued a temporary license by default. When a client computer or device connects to a terminal server for the second time, if the license server is activated and enough TS Per Device CALs are available, the license server issues the client computer or device a permanent TS Per Device CAL.
A Per User CAL gives one user the right to access a terminal server from an unlimited number of client computers or devices. TS Per User CALs are not enforced by TS Licensing. As a result, client connections can occur regardless of the number of TS Per User CALs that are installed on the license server. This does not release administrators from the requirements of the Microsoft Software License Terms to have a valid TS Per User CAL for each user. Failure to have a TS Per User CAL for each user, if the Per User licensing mode is being used, is a violation of the license terms.