Shined up real nice.
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The slash filter works by examining story descriptions and hiding things which contain words like "slash" as long as they don't also match an exceptions list (eg. "not slash").
That's the best I can do on FFnet because of their limited metadata compared to other fanfic hosting sites.
As for hiding het, it's not reliably possibly at the moment but could be done. I'd have to add a checkbox to invert the filter so it hides non-matches rather than hiding matches. (Since people generally only bother to mention that something's non-slash in their description when the summary may give the wrong impression.)
At the moment, it doesn't have support for applying a filter to the pairings field since I've been low on time and have been prioritizing features I'd actually find useful.
In my experience, the pairings field on FFnet isn't very helpful for two reasons:
First, because I'll read just about any pairing as long as it's well-written. (I only filter slash because so many slash authors are willing to sacrifice quality in order to make their pairing work despite a total lack of canon evidence that the characters would be attracted to the same sex and a lack of willingness to spend enough time building the relationship to get over the social conditioning against that kind of exploration.)
Second, because it's not really a pairing field. It's more a "these are the main characters" field, which makes it even more useless to me, since "Harry X Draco slash" and "Harry vs. Draco battle to the death" mean completely different things but they both show up as "Harry/Draco".