I don’t know that it can be well-defined. It might be asking something akin to “What makes something human?” For usability, one needs a sense of what defines “user pain” and what defines “reasonableness.” No product is perfect. They all have usability problems at some level. The best usability experts, and tech writers who do this well, have an intuition for user priorities and an ability to identify and differentiate large usability problems from small ones.

Thinking about this some more now, I can imagine a future in which we'll see more and more software for which AI agents are the main users.

For tech documentation, I suppose that AI agents would mainly benefit from Skills files managed as part of the tool's repo, and I absolutely do imagine future AI agents being set up (e.g. as part of their AGENTS.md) to propose PRs to these Skills as they use the tools. And I'm wondering whether AI agents might end up with different usability concerns and pain-points from those that we have.