https://dart.dev/language/class-modifiers
Class modifiers
Modifier keywords for class declarations to control external library access.
class modifiers
https://dart.dev/language/class-modifiers-for-apis
Class modifiers for API maintainers
How to use the class modifiers added in Dart 3.0 to make your package's API more robust and maintainable.
class modifiersapimaintainers