Bugs are errors in the software, i.e. programming faults. If something is not working the way it should, testers speak of Bugs. These problems have their root in technical implementation. If a function is not working or is displayed incorrectly, even though it was implemented, there is a Bug.
On the other hand, it is a usability problem, if useful functions are in an unexpected place or missing completely. Using the software becomes difficult or even impossible. In any case, usability problems make using the program tedious and uncomfortable. They should be mentioned in your test report as "Negative Feedback", not as Bugs. So if there are functions that you find important, but which aren’t included in the software, it’s not a Bug but a usability problem.
Example 1: The text is too small
It’s a Bug if: There’s an option in the menu of the app to change the text size. After pressing the button nothing happens.
It’s a Usability problem if: The text size of the app is very small, but there is no option in the app to change it.
Example 2: You can’t put an appointment in the calendar
It’s a Bug if: The appointment you entered isn’t saved.
It’s a Usability problem if: The text field to enter the appointment is too small.