That’s simply the section that limits the number of search results displayed. To actually use it, also look under Tools> Batch Cat
Here you can perform (very) simple searches, see the posts that show up with a text search, check them all and then batch add category(ies) for each of those posts. You can also batch drop.
Or by including no search text, you can show all the posts under one category and add them to another category.
No idea how to do more complex boolean esque searches yet myself… The default batch post edit functionality in wordpress can do quite a bit of this, but it has been my experience that the core functionality code is very heavy and if you are doing lots of edits (hundreds of posts with mulitple category changes) it can crash a site pretty quick. this is a lot lighter.
