In a Dash app, the server attribute in app.server corresponds to a Flask instance. So if you were looking at a Flask tutorial (such as the Flask Quickstart guide), @app.server.route would be likely to be presented as @app.route. It’s a little confusing, but the variable name app is simply a convention. In Dash apps people typically use it to contain a Dash app (with a Flask instance on the server attribute), whereas in Flask apps app usually contains the Flask instance.