Each time you reference app-globals in a custom component, a new instance of app-globals is created. But each of these instances can share a number of properties (think static vars in Java or class vars in ObjC / Swift).

The Script within app-globals element (or indeed any Polymer element) runs only once - think of it as running when the component definition is loaded. But the Polymer function within that script declares a configuration object, with properties and lifecycle event-handlers, that will be created separately for each instance. The app-globals script in the document you reference (copied below UPDATE: this version is modified as described later) uses an anonymous closure (a function that is run immediately), containing both the shared variables and the Polymer function declaring the config object which in turn references the shared variables. So each instance of that config object - and in turn each instance of app-globals - will reference the same set of shared variables.

 <polymer-element name=app-globals>

  <script>

  (function() {

    var data = {

      firstName: 'John',

      lastName: 'Smith'

    }



    Polymer('app-globals', {

       ready: function() {

         this.data = data;

       }

    });

  })();

  </script>

</polymer-element>

If one custom component instance changes a value on its app-globals instance (or they are changed internally, as the results of an AJAX call in your case) all other component instances with a reference to app-globals will see the changed value.

UPDATE: The original, as copied from:

http://www.polymer-project.org/docs/polymer/polymer.html#global

had a deficiency, as described by @zreptil, if you change the data values, the new values are NOT available to all other instances - because the instance variables are just copies of the referenced strings. By using an object with data properties, as in the edited version above, and only ever reading from and assigning to the data properties of that object rather than overwriting the object itself, changed values are shareable between instances.