The Windows installation ISO contains multiple Windows image files (WIM):
- boot.wim - this is used to boot the Windows Preinstallation Environment (WinPE) for a clean install
- install.wim - this contains the image(s) that will be installed.
The Windows Server 2016 ISO contains multiple versions in this single install.wim file which is why the same media can be used to install Standard or Datacenter editions, with the desktop experience or without it (core).