Example: the client licenses for Windows Server cal 2008, you can connect to Windows Server 2003, but not vice versa, the exception – for Windows Server cal 2003, you can connect to Windows Server 2003 R2. Now suppose that one of the servers installed Exchange Server 2007, respectively, will have to purchase a license to own Exchange Server and Exchange Server cal for up to five PCs. Another point: the supply of oem and boxed versions of client licenses included delivery of 5 pieces for corporate licenses, olp buy more separately. There is another option licenses – Terminal cal – license designed for terminal access. Try to understand the example: Suppose a company has decided to expand its fleet of PCs and buy another 5 thin clients (terminals) to save money and space (for more information about thin clients can be found online). In this case, companies have to buy another 5 clients a Windows Server cal and 5 Terminal cal, because the latter, qualify only for connecting the terminals. CALs are of two kinds of "user-friendly (user CAL) and a device (device CAL). The most common client license per device, then has licensed the device to connect to the server, and many employees can connect with this device to the server. The second option per user license "is beneficial in cases where the number of employees who need access to the server is limited, say, 20 PCs, and access needed only two employees. In this case, would be advantageous to purchase 2 client licenses, and these two officers will be able to connect to server any pc, but only these two members, which includes a license for the user.