After a while, I went into my mail logs and after looking at items caught by Sniffer, I guessed that it was just as you explained; that Sniffer just adds to the total of any other regular SmarterMail checks.
Just in case of possible false-positives with Sniffer, I set its value at one less than my delete threshold. That way if another test also catches it, the total will cause deletion; but just one less will through it into a spam folder so user can check.
Good to hear you say that MS has very few false-positives or none. After a bit of hearing from my users, I may go back and set it higher.
I realize my settings are much higher than is usual or recommended. Probably something faulty in my overall set of settings, but I found I had to be that strict or spam was abundant. Using the setting often recommended, did not seem to help. Actually, even a lot with the settings I show until I tried SM, folks still getting a lot of spam. It seems great!