The name of the model class to query against. This can either be a model name (i.e. 'Users'), or a fully-namespaced class reference (i.e. 'app\models\Users'). When authenticating users, the magic first() method is invoked against the model to return the first record found when combining the conditions in the $_scope property with the authentication data yielded from the Request object in Form::check(). (Note that the model method called is configurable using the $_query property).