Valid metrics for licensing Microsoft products are based on the general and specific Product Use Rights (PUR), which are published by Microsoft every three months. This means that license models that are valid for the product version in use primarily depend on these product use rights.
Licensing per processor core is also subject to the separately published Core Factor Table that is applied to the number of cores of physical processors. This is not applied to virtual machines.
However, certain master contracts (e.g., Enterprise Agreement) require customers to base usage on respective product use rights.