What is that?

John, setting aside that questions attached to specific lessons have answers restricted to the lesson material, I think you are asking more generally about the difference in use of ‘quel’ in specific and general questions. Hope that is correct. 

My understanding (stand to be corrected) is that ‘quel’ is used when asking for specific information about a ‘known’ thing - the time, the date etc (fixed expressions), or which one (of), what or which type/sort/nature/quantity of etc 

This will often be in the context of an explicit choice - eg ‘which man ?’. Or the question “Which is the  Panthéon ?”

However, it can be with either an implicit choice, or the possibility that a choice exists - when describing ‘which (what) is the best class ’, or ‘what (which) is the solution’.

On the other hand, ‘quel’ is not used in the general case, when seeking general information about an ‘unknown’ thing  - ‘What is the Panthéon ?’ 

I think these distinctions are harder for English speakers because ‘what’ is used for the general case and also frequently for the specific, with ‘which’ restricted to the specific case. 

https://www.lawlessfrench.com/grammar/interrogative-adjectives/

https://dictionnaire.lerobert.com/definition/quel 

https://www.larousse.fr/dictionnaires/francais/quel/65610 

https://www.dictionnaire-academie.fr/article/A9Q0167

Agree with Maarten. The pronouns quel (and its various forms) as well as (lequel) are generally used to decide within a given group of things.

Parmi ces bâtiments là, quel est le Panthéon? -- Among those buildings, which (one) is the Pantheon?
Qu'est-ce que c'est le Panthéon? -- What is the Pantheon?