rest\UrlRule $controller

The controller ID (e.g. user, post-comment) that the rules in this composite rule are dealing with. It should be prefixed with the module ID if the controller is within a module (e.g. admin/user).

By default, the controller ID will be pluralized automatically when it is put in the patterns of the generated rules. If you want to explicitly specify how the controller ID should appear in the patterns, you may use an array with the array key being as the controller ID in the pattern, and the array value the actual controller ID. For example, ['u' => 'user'].

You may also pass multiple controller IDs as an array. If this is the case, this composite rule will generate applicable URL rules for EVERY specified controller. For example, ['user', 'post'].