Perhaps 'Filters' was used to suggest some kind of multiple prefix or multiple filter option. I don't mind the use of the word filter. For anyone who has done any form of online shopping, they'd understand what filter meant.
This is a very good feature suggestion imo. Additionally, I believe whole prefix groups should be selectable.
Example: Language prefixes such as Norwegian and Swedish. The languages are so similar they can be understood in either direction, so it would be beneficial for someone to see threads with either prefix.
Multiple prefixes does sound interesting as well, although perhaps tags would work better in that area.