I just discovered that you can explicitly use the word "constructor" when defining a class; and that is mandatory only when specifying a visibility modifier for the constructor, or when using an annotation. Hence, I am curious about what annotation one could use for a constructor and why. Many thanks.

If you use dependency injection framework like Dagger you can use

@Inject

annotation for constructor to mark that this constructor is injectable.

or https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.jvm/-jvm-overloads/index.html