It's also about the strongest stout you can buy for the money, at 10.8 percent. The combination of dark chocolate drinkability with high gravity alcohol kick is more than a little dangerous to the unwary, so approach it with caution. You could easily have too much of this stuff before realizing you were doing so.
A few weeks back, I mentioned that I didn't really know the difference between a stout and a porter. Well, now I do. All stouts are porters, meaning dark black ales. Historically, porter brewers would refer to their strongest porter as “stout” or “strong” porter. Since one company's stoutest porter could be another company's weakest, this was never really a consistent or logical term, but there's a vague and general tendency for the word “porter” to be used for weaker black beers and the term “stout” for stronger ones. To all intents and purposes, though, they're the same type of beer. Either a porter or a stout can be “Imperial,” “Russian,” or “Baltic,” which generally indicates a stronger beer. Nobody would call a 10.8 beer like Night Tripper a porter, but then again there are stouts and even “Imperial” stouts that aren't particularly strong. So there's really no rule, but broadly speaking you could say that a porter should usually be like a stout without too much alcohol, and a stout should be like a stronger porter.