Framework Documentation - Router Package

How to Pass Extra Information from a Route to a Controller

The Joomla Router allows parameters inside the defaults collection that don't have to match a placeholder in the route
path. This means, you can use the defaults array to specify extra parameters that will then be accessible for use in
your Request object or controller.

use Joomla\Router\Router;

$router = new Router;
$router->addRoute(
    'GET',
    '/user/:id',
    'UserController@show',
    array(
        'id' => '(\d+)'
    ),
    array(
        'id': 0,
        'username': 'Fred'
    )
);

As you can see, the username variable was never defined inside the route path, but you can still access its value from
the return once the URL is parsed.