> Visible to whom?

The user. Your tests are your contract with the user. Any time there is a user, you need to establish the contract with the user so that it is clear to all parties what is provided and what will not randomly change in the future. This is what testing is for.

Yes, that does mean any of classes, network services, graphical user interfaces, etc. All of those things can have users.

> Unit is the lowest level of abstraction and e2e is the highest.

There is only one 'abstraction' that I can see: Feed inputs and evaluate outputs. How does that turn into higher or lower levels?