[Supperman]

Hi.
Could you explain why the definite article must be the definite article, instead of the indefinite article?

(context) If you go to a new restaurant, and you want to go to the restaurant's rest room, you would ask the staff;
"Where is the rest room?"
(Not "Where is a rest room?")
(Not "Where is your restaurant's rest room?")

You and the staff didn't speak the topic of "the restaurant's rest room" before.
If you knew where the rest room was in advance, you would never ask the question.

Why the definite article?
What is the function of the definite article here?

Thank you.