It's all in the link I posted
Per User or Per Device Mode
Per User/Per Device mode is defined as follows:
? A separate Windows CAL (of either type) is required for each user or device that accesses or uses the server software on any of your servers.
? The number of Windows CALs required equals the number of users or devices accessing the server software.
? If you choose this licensing mode, your choice is permanent. You can, however, reassign a Windows CAL from one device to another device or from one user to another user, provided the reassignment is made either (a) permanently away from the one device or user or (b) temporarily to accommodate the use of the Windows CAL either by a loaner device, while a permanent device is out of service, or by a temporary worker, while a regular employee is absent.