Questions about OpenOffice Help System

[HelenYue]

Hi, everyone:

I am new to OpenOffice. Currently I am studying the OpenOffice source code. One finding is that the help document is actually xml files. I think it should be easy to extend, but I found something strange in the .xsl file. Some url is parsed as: vnd.sun.star.help://@xlink:href?....
It is said that it is used for the tag: embedded & popup-cut. Could anyone tell me what is the meaning of vnd.sun.star.help and these two tags?

I also found the index direcotry *.idx is used for the search engine. Could anyone tell me some resource about the indexing engine? It is strange that I found no document about that.

Thanks in advance.