This article is for informational purposes. ROK licenses are legally bound to specific hardware (usually servers from Dell, HPE, or Lenovo). How to Download Windows Server 2019 Standard ROK (16-Core) ISO If you have purchased a server from an OEM (like Dell, HP, or Lenovo) that came with a Windows Server 2019 Standard ROK (Reseller Option Kit) license, you do not download the software from Microsoft’s public website. You must use a specific channel.
Here is the official guide to obtaining your ISO and product key. A Reseller Option Kit (ROK) license is pre-installed on new server hardware by the manufacturer. Unlike a retail license (which you can move between servers) or a Volume License (VL), an ROK license is tied to the specific hardware it was purchased with.
The "16-Core" specification refers to the minimum licensing requirement for Windows Server 2019 Standard (all physical cores in the server must be licensed, with a minimum of 16 cores per server). Your download location depends on who made your server. You cannot get the ISO directly from Microsoft.com using an ROK key.