basic principles, how people are the most immutable part of the system, around distraction, etc.
If you design systems, you are familiar with constraints. For example, it is hard to add new fields to a database; mobile networks sometimes provide poor connectivity or have slow performance or high latency. Typically, system design takes such constraints—and, of course, costs—into account in determining what a team can build.
But, as UX professionals, it’s generally our goal to build products that people can use. At the core of user-centered design is the understanding that users are our biggest resource constraint—and the hardest thing to change.
We don’t train people to be part of a system, so we have to take them as they are. However, because we do control the rest of the system, we must do our best to take people into account.
I like the HF term because of how it's practiced. It's not a single bit like most digital UX, dealing with the UI of a system, but is anything human facing. HF engineers for aviation work on training, and signs, and documents, and a hundred things outside the buttons, switches and lights on the cockpit control panel.