Since Windows Server is licensed based on "physical" cores on the physical host/server, the physical server hardware needs to be licensed and reported by the SPLA provider in a multi-tenanted or shared hardware environment. At this stage, Windows Server do not have any "license mobility through Software Assurance" right - I.e. the end customer cannot bring their Windows Server licenses to your shared hardware/multi-tenanted environment.
However, end-customer can bring their volume licenses to be hosted on a dedicated hardware. In this scenario, there are no requirement for active Software Assurance on customer’ licenses. Note that, from a Microsoft perspective, the term “dedicated” refers to the physical server hardware that is dedicated solely for the end-customer.