What you're referring to is searching within categories in a subdomain. HP is its own subdomain, books is a subdomain, buffy is a subdomain, and so on.
Due to the size of the database itself, as well as the current search engine limitations, you CANNOT search for terms of less than 4 characters. Not only that, the current search engine limits results to no more than 200 per search.
As to what you're specifically looking FOR, if data is not entered to enable your search term, in the summary, you won't get a result anyway, no matter if you're using the current engine, or when we finally get the new one integrated in.
It would be helpful if I knew precisely what you're wanting to find, because then I can likely help you structure your search terms to work more effectively. On the other hand, there are instructions at the search page, to do exactly that.
The DOMAIN is adult-fanfiction.org. The SUBDOMAIN is hp.adult-fanfiction.org. Searches are done within a subdomain for the specifics. For example, if you were to search for hp related stuff in books, xmen, or any other subdomain but hp, you would find nothing at all.