Difference between #! and ! in Ionic href


Inherited a code, where href is defined as href=#!/page1/param1, as opposed to href=#/page1/param1. By the way, #/page1/param1 does not work in this setup, where as the “#!” works.

What is the difference between both? And is this any reason why my Back buttons do not even show up in the pages?

The state definitions in config file are,
.state(‘app.page1’, {
url: ‘/page1/:param1’,

  views: {
    viewContent: {
      template: require('./page1.html'),
      controller: 'PageOneCtrl'
    } /* end of viewContent */
  } /* end of views */

}); /* end of state app.page1 */