Yii2 model scenarios can be used differently. Lets say, we have a model as Customer 
Which can be used to store customer sign-in inputs, 
but it can be also used for the customer sign-up purpose. 
In different scenarios, Yii2 model can be used in multiple business rules and logic.
the email attribute may be required during customer sign-up, 
but not required customer sign-in.

Yii2 model uses the yii\base\Model::$scenario property to track the scenario. 
By default, Yii2 model supports only a single scenario as default scenario. 
Below code shows two ways of setting the scenario of Yii2 model on controller section:

$model = new Customer;
$model->scenario = Customer::SCENARIO_LOGIN;
$model = new Customer(['scenario' => User::SCENARIO_LOGIN]);

in many cases, we need a rule to be applied only in particular scenarios. 
To do so, we can specify the rule property as like below example:

in model file:
public function rules()
{
    return [
        ['title', 'required', 'on' => 'create']
    ];
}

in controller file:
public function actionCreate()
{
     $model = new Item();
     $model->scenario = 'create';
     ...
}

Tags : PHP, Yii, Yii2,