By default Yii::t(string $category, string $message …) translate function accepts some parameters and first of them ($category) is message category. It can be defined as single word or two words with dot beetween (for widgets and modules).
But I have found that you can define $category as ‘module/user’, and then Yii will be search for translation files in /protected/messages/#your_language#/module/user.php. That can be very nice to keep translation files organized.
For example, using this code
Yii::t('model/user', 'Lorem ipsum');
Yii will look for translation files in
/protected/messages/#your_language#/module/user.php