User Permissions
Modules and plugins can register new user permissions to the system using the EVENT_REGISTER_PERMISSIONS (opens new window) event:
use craft\events\RegisterUserPermissionsEvent;
use craft\services\UserPermissions;
use yii\base\Event;
public function init()
{
    parent::init();
    Event::on(
        UserPermissions::class,
        UserPermissions::EVENT_REGISTER_PERMISSIONS,
        function(RegisterUserPermissionsEvent $event) {
            $event->permissions['Permission Group Name'] = [
                'permissionName' => [
                    'label' => 'Permission Label',
                ],
            ];
        }
    );
}
Permissions can also have nested permissions by adding a nested key to the permission array.
'permissionName' => [
    'label' => 'Permission Label',
    'nested' => [
        'nestedPermissionName' => [
            'label' => 'Nested Permission Label',
        ],
    ],
];
Nesting is meant for UI only; if you wanted to reference nestedPermissionName in the example above you would use exactly that key.
#Requiring Permissions
Controllers can require that the logged-in user has a permission by calling requirePermission() (opens new window).
public function actionStayUpLate()
{
    // Require the `stayUpLate` permission
    $this->requirePermission('stayUpLate');
}
If the user doesn’t have that permission, then a 403 error will be returned.
Templates can also ensure that the user has a permission with the requirePermission tag:
{% requirePermission 'stayUpLate' %}
#Checking Permissions
You can check if the logged-in user has a permission by calling craft\web\User::checkPermission() (opens new window):
// See if they have the `stayUpLate` permission
if (Craft::$app->user->checkPermission('stayUpLate')) {
    // ...
}
You can also see if any given user has a permission by calling craft\elements\User::can() (opens new window):
/** @var \craft\elements\User $user */
if ($user->can('stayUpLate')) {
    // ...
}