Earlier: "... I want to add keywords (tag/annotate) to [an article] and retrieve those ... on another page ... the problem ... the result would be an article. I need only a particular section of the article as result ..."

Peter Blaise responds: I broke out each section to it's own sub page, and that may help you get, in your search results, what was once only the former "section", but because now it's on it's own page, it is a discreet nodule of information that responds better to such searches and links on it's own as a separate result. But then, how to reassemble and display the content of all these new subpages back together again on the main article page above those sub pages so they look and read as you originally intended?

http://www.mediawiki.org/wiki/Extension:SubPageList2

... with:

<subpages deepness=1 mode=preview />

... in the content of article page above all those subpages (along with any other content I want, such as automatic or my own construct of a table of contents of the sub pages, and more). What were previously sections now have all the features and benefits of independent pages, yet the extension, SubPageList2, permits me to display them as if they were still sub sections, all on one page. This gives me a pleasant selectable mix of page, section, and subpage features and benefits from which you may be able to design a solution to your challenge.

There's more. This allows people to discuss/talk about what looks like a section of a large page, but they get a separate, dedicated discussion/talk page for their "section" because it's really a page on it's own. Otherwise, when we try to discuss/talk about a section on a page, we get the discussion/talk page for the entire master page, not exclusively for that sub section on it's own! Now, each section has it's own discussion/talk page- cool! Note, I "protect" the article page so people don't waste their time trying to edit an unassembled ghost. When they click on what looks like a sub section, it brings them to a dedicated sub page for that section, and then they can edit or discuss/talk about only that "section".

Let us know how you resolve your design challenge. I really enjoy these architecture discussions. Sort of like opening a can of paint with a screwdriver, in that it's not in the instruction manual for the screwdriver. That's the way we seem to use the Mediawiki tools, too - very creatively, and far off the official, simplistic "instruction sheet" that comes with each extension!

Thanks to Martin Schallnahs and Rob Church for SubPage2.