For connections between HMIs and higher level computing systems, Ethernet is one part of the equation along with myriad other standards. For purposes of this discussion, an HMI is defined as a computer hosting packaged software running on a Microsoft or a Linux operating system.
HMI/higher level computing system connectivity standards can be divided into three levels. First, there are physical hardware connection standards, nowadays almost always Ethernet. Second, there is the particular Ethernet software protocol, typically TCP/IP at this level.