What does '@' sign in $stateProvider used for in managing navigations

ionic-v1
1

Hello every one i am study already written code in ionic. in navigation ` @ sign is used. i want know what is this sign used for? code snipet is given below

 }).state('app.categories.deals', {
        url: '/:catId?title',
        views: {
            'menuContent@app': {
                templateUrl: 'templates/deals-list.html',
                controller: 'CategoryDeals'
            }
        }
    }).state('app.categories.deals.detail', {
        url: '/:dealId',
        views: {
            'menuContent@app': {
                templateUrl: 'templates/deal-detail.html',
                controller: 'DealDetail'
            }
        }
    }).state('app.related-deal-detail', {
        url: '/related-deal-detail/:dealId',
        views: {
...

anyone please?