Microsoft SQL Server license has lots of flavors in terms of user based \ CAL or processor based license, license based on versions or license based on environment (physical or virtual).
Although, Microsoft release license guidelines for each & every SQL server version. But there are more complex scenarios for licensing then we think.
Best person to answer your all queries over licensing is MICROSOFT itself.
You can call Microsoft Licensing at 1-800-426-9400, Monday to Friday, 6:00 A.M. to 6:00 P.M. (Pacific Time) to speak directly to a Microsoft licensing specialist.
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Reference : Rohit Garg (http://mssqlfun.com/)