 <p>Square brackets means that it's an array (basic javascript). Your require array only contains 1 item now, but it can contain more items. Your controllers is an array of 2 items. </p> <p>In your define example xtype doesn't expect an array but a string. Same with extend, you can only extend from one component. </p> <p>This info can be found in the docs of ExtJS aswell. For example the controllers config from your example above:</p> <p><a href="http://docs.sencha.com/ext-js/4-1/#!/api/Ext.app.Application-cfg-controllers" rel="nofollow">http://docs.sencha.com/ext-js/4-1/#!/api/Ext.app.Application-cfg-controllers</a></p> <p>The docs mention this <code>controllers : String[]</code>. Which means it expects an array of strings.</p>