Segue parte de um KB: (Fonte: Guidelines dor Deploying Win2k3 - Microsoft site)
5. Deciding What Type of Client Access License (CAL) to Purchase
When you have completed all the preceding tasks, Windows Server 2003 Terminal Server can operate in two licensing modes: Per Device (default factory setting) and Per User. By default, Terminal Server Licensing is configured in Per Device mode.
A Per Device CAL gives each client computer or device the right to access a terminal server that is running Windows Server 2003. If you install Per Device CALs on your license server, you need to ensure that the licensing mode on Terminal Server is set to Per Device.
Using Per User licensing, one user can access a terminal server from an unlimited number of devices and only one CAL is needed instead of a CAL for each device. If you purchase and install Per User CALs on your license server, you must set the licensing mode on to Per User.
Note: Per User Licensing is not monitored by Terminal Server. This means that when you install Per User CALs, do not expect the number of available Per User CALs to be monitored."
Também segue link para ajudar a escolher o tupo de call que melhor se adapta a você:
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