An object extension is a single object, used for visualization, geo-mapping, selections etc. It is mostly treated like any other object and is mostly used for 3rd party charts/visualization. It can be added by right-clicking inside AJAX/WebView and add extension object.
A document extension is something that targets the whole site, add whole functionality to the site (visually (injects CSS) or functionally (e.g. Firebug, send variables, connect elsewhere, override default functionality)). Document extensions are added by the document properties inside QlikView Desktop.
Both types of extensions only works in AJAX/WebView and are installed the same way, but in different folders (Documents vs Objects).
I've wouldn't wanted to start a new question as probably is a quite basic issue and I am just missing something from the order of the steps. I have gone through of a article about how to install and use extensions; found on the Qlikfix.com. -"QlikView Extension tutorials, documentation and examples" and installed Alexander Google Map - Cluster extension. It seems after running the qar file that the extension installed. It is in the correct path and no error message appeared but when I am trying to use any extensions installed (I tried couple of more) even if I switch to WebView, just haven't got the proper option to use new sheet objects as an extension. Can I have someone's advice please? It is an local PC but not personal edition.