signupform下的targetClass是什么 意思。

高级版的 frontend/models/signupform里有段代码

public function rules()

{
    return [
        ['username', 'filter', 'filter' => 'trim'],
        ['username', 'required'],
        ['username', 'unique', 'targetClass' => '\common\models\User', 'message' => 'This username has already been taken.'],
        ['username', 'string', 'min' => 2, 'max' => 255],

        ['email', 'filter', 'filter' => 'trim'],
        ['email', 'required'],
        ['email', 'email'],
        ['email', 'string', 'max' => 255],
        ['email', 'unique', 'targetClass' => '\common\models\User', 'message' => 'This email address has already been taken.'],

        ['password', 'required'],
        ['password', 'string', 'min' => 6],
    ];
}
----------

 ['username', 'unique', 'targetClass' => '\common\models\User', 'message' => 'This username has already been taken.'],
 ['email', 'unique', 'targetClass' => '\common\models\User', 'message' => 'This email address has already been taken.'],
 
 这上面两行有  'targetClass' => '\common\models\User',  --这句话是什么 意思？
 我在user这个文件里，找不到相关的内容。