Does XML covers the representation of any structured data and knowledge I see nowadays XML is used to structure any file or data. It can represent both flat and hierarchical relationships. For example, it can be used to show the parse tree of a sentence in a natural language.

I am not familiar with different types of data and knowledge structures, but I would like to know if XML is theoretically the ultimate representation scheme which can model and structure any data and knowledge?

What are the reasons behind the popularity of XML?
XML is nothing more than a well-defined way to store trees of strings. Since even plain strings can encode everything you can encode in practice (i.e. countable sets), yes, XML can "model" everything. 

But that's nothing special.

The popularity of XML is probably due to


it being standardized and
the amount of tool support that has developed.


There is no information-theoretic miracle involved.