Human-machine interaction happens not just for interaction. It has some purpose. And this purpose is defined by subject domain — or subject matter — in which the activity happens.
Subject domain is defined by certain activities, that are surrounded by industry standards, rules and traditions, specific knowledge and so on.
Examples of subject domains are medicine in general and neurosurgery in particular, shipbuilding and maritime navigation, engineering and software development and so on.
One major thing to concern is that users do some activities by some rules, defined in subject domain. And these activities and rules are not necessarily related to user interfaces, software and hardware. Machines are just tools that help users to achieve their goals and tasks faster and with less efforts.
This means, that user interest is shifted from tools to activities and results — so the tool itself is not so really important for the user, as how it fits his needs, habits and processes.