When you may reassign a licence | License Mobility in Server Farms

What does the "licence reassignment" rule in Microsoft Licensing have to do with virtualisation? Have you ever wondered about the mysterious "90-day rule" that licensing managers like to talk about? Do you know when you may reassign a Microsoft licence from a device to a device or from a user to a user? And why is License Mobility in Server Farms so utterly important when you virtualise your servers?