what does abstract:true do in ui-router?

Question

Sorry my question may seems duplicate beacuse i'm learning Angular JS with some downloaded template but i dint get anything what i exactly need with my searching.

what does abstract: true means?

the main question is .otherwise('/app/dashboard'); taking me to partials/app_dashboard.html if i had mentioned /dashboard in otherwise it should take me there but why /app/dashboard landing me in partials/app_dashboard.html. does abstract: true have any effect in otherwise?

header div, container div(ui-view) and footer div was placed in app.html

$urlRouterProvider

        .otherwise('/app/dashboard');
    $stateProvider            
        .state('app', {
            abstract: true,
            url: '/app',
            templateUrl: 'partials/app.html'
        })
        .state('app.dashboard', {
            url: '/dashboard',
            templateUrl: 'partials/app_dashboard.html'
        })
        .state('app.ui', {
            url: '/ui',
            template: '<div ui-view class="fade-in-up"></div>'
        })

Answer 1

abstract - {boolean=} - An abstract state will never be directly activated, but can provide inherited properties to its common children states.

From ui-router docs