HI, Anil.
The best way to learn about this is to have a look at this post.
Extension objects often provide a utility not available within QlikView - shapes, patterns, maps, charts etc. However, the downside is that it may not give you all the features that a QlikView object would hold such as drilling through or manipulating the data. It also may not be technically supported by Qlik.
Extension Object Basics.doc
Cheers,
Neil
