A legal authorization to use software in a given networking scenario. Merely purchasing Microsoft Windows NT, Windows 2000, or any other Microsoft BackOffice software does not legally authorize you to use this software in a given networking scenario; you must also have the appropriate license. You generally obtain a server license for each server and a client access license (CAL) for each client that accesses the server.
In the BackOffice licensing model, each server and client computer is licensed separately. Each BackOffice server requires a server license. The server license is included when you purchase the particular BackOffice product, and it grants you the right to run that product on a particular computer.
Each client connection to a BackOffice server requires a CAL, a legal document that grants a client machine the right to access the services provided by a BackOffice server. CALs are generally not included when you buy your BackOffice software and must be purchased separately.
Every client machine, no matter what its operating system (Windows, Macintosh, UNIX, and so on), must have a CAL to access either basic Windows NT or Windows 2000 networking services such as file and print services, services for Macintosh, services for NetWare, or Remote Access Service (RAS) or to access any BackOffice application, such as Microsoft Exchange Server.