I am in the middle of doing your writing exercise "Castile comes from 'Castle'" > https://progress.lawlessspanish.com/my-languages/spanish/exercises/take/667/6083595?page=6 ... and incorrectly used "¿Qué ... ?" to translate "What is its origin?" ... I have to confess that the difference between "¿Qué es ... ?" and "¿Cuál es ... ?" was something which I had not yet grasped properly !
I consulted my grammar book, which explains that "¿Qué es ... ?" really means "What kind of thing?", i.e., inquiring about the definition of something's nature.... > which is of course what you say in your reply [below] to R.Z., [Thank you Inma !] - and which is supplemented by the other comments from Kwiziq students.
It might be worth mentioning this distinction in the notes accompanying the above-mentioned writing exercise.
