Basically, removing the 90 day re-assignment rule such that workloads can be moved more freely within a Server Farm (details in the doc) 

excerpt from http://download.microsoft.com/download/3/d/4/3d42bdc2-6725-4b29-b75a-a5b04179958b/Application_Server_License_Mobility_VL_Brief.doc

 "With the changes, both licenses and software can move more freely across servers in a server farm, potentially reducing the number of licenses needed to support your workloads. Effectively, the changes mean instead of counting instances or processors and licensing by server, you are able to count instances or processors and license by server farm.  "