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Special trailer for Triple 9 is one half preview, one half featurette.
Film tells the story of a gang of criminals who, with the help of an inside man, attempt to pull off a major heist. The only problem is that the only way they can feasibly get away with it is to create a Triple 9, by taking down a police officer to draw away other officers.
The official code definition of 999, Triple 9, used by law enforcement is "Urgent help needed / Officer down".
The "Unthinkable" trailer for Triple 9 is the second trailer of the day to mix the concept of being a trailer with interviews with cast and crew; the definition of a featurette. Our assumption is that this particular Triple 9 trailer was shown in theaters before the start time as a special preview of the film. Sometimes this segment is called "The 20" or some other name for the period of advertisements before the real trailers begin.
Though this format of trailer is not typical, it works extremely well in this case. Going into detail about the story and having the [strong] ensemble cast better explain what drew them to Triple 9 works surprisingly well at convincing moviegoers that the film is worth the trip to the theaters.
So much for the last trailer being the "final trailer".