Hmmm. Not sure I understand your question. If you click SEARCH, then click ADVANCED SEARCH, it allows you to select ALL FORUMS, or select the particular forum in which you wish to search.
As an example, right now I am within the "Getting Started on the Board" forum. I click SEARCH, then ADVANCED SEARCH, then specify the keyword "refrig", and specify the "Appliances" forum. It returns just what I expect - all the posts in the Appliances forum (and nowhere else) that contain the word "refrig". Of course, there are a ton of 'em.
If I repeat the search for "refrig", but specify the "Tires tires tires" forum, again I get just what I expect - nuthin'.
BTW, I always use ADVANCED SEARCH. The plain-vanilla SEARCH is much too broad, since it doesn't allow you to specify a date range, a poster's name, a forum ...
Bill