A question came up the other day regarding installing ESU licenses on a shared platform if the service provider (under a SPLA agreement) is licensed for Windows Datacenter but installed Windows STD VM. That’s pretty common and though I would address it today.
One should not be confused over what is installed v what is actually licensed. As the SPLA Provider is licensed for Windows Server Datacenter, the server should be covered with ESU Datacenter. Running a Windows Standard VM when licensed for Windows Server Datacenter does not change the license requirement.
It is also important to note that ESU is not available in SPLA, but is available through CSP, EA, and SCE agreements. The Service Provider will not be licensing ESU, but the end customer. Think of it as license mobility without the need for Software Assurance.
I would recommend checking out the Product Terms page 93. https://www.microsoft.com/en-us/licensing/product-licensing/products