Examples of application state:

User preferences

Login info

Notifications in a social networking app

The shopping cart in an e-commerce app

Read/unread state of articles in a news app

For managing app state, you’ll want to research your options.
Your choice depends on the complexity and nature of your app,
your team’s previous experience, and many other aspects. Read on.

There is no clear-cut rule

To be clear, you can use State and setState() to manage all of
the state in your app. In fact, the Flutter team does this in many
simple app samples (including the starter app that you get with every
flutter create).