May also be used by someone warning a new member about a policy they don't necessarily agree with so they can clean up their act before us mean old admins can get to them. Either way, it's used almost exclusively by people who are misrepresenting the moderators/admins/policies of NH.
To be fair though, in ye olden days of harshness with the old regime, new members would have to be quickly warned by other members, because for a good while there was pretty close to a 1 strike policy. I distinctly reclaim warning several people myself, usually about posting without a picture in the pictures thread, although it was pretty much 50/50 whether they'd see my message before the mods saw their post.
Granted, I'd almost always do this via PM, rather than a post, so the filter thing was irrelevant.