Why we need xalan extension function in XSL ? In which scenario its required to implement ?

Extensions are there to provide things that standard XSLT doesn't provide.

Other than that I'm not quite sure what you mean by 'required to implement'? Are you writing an XSLT processor?

/- Sam Judson : Wrox Technical Editor -/

I am working with XSL to create the ASCII file. I need to sum up the lines (element1,element2). I just planning first should produce an node set - using xalan extension function before to be outputed... Any idea ?

Can I suggest you start by reading the post on "What makes a good XSLT Post": http://p2p.wrox.com/topic.asp?TOPIC_ID=49630

Then try describing your problem, what your input looks like, what you want your output to look like and what you have tried so far.

/- Sam Judson : Wrox Technical Editor -/