In this Daily Chess game, I moved Nc3 which is the Slav Defense: Three Knights Variation. Then, curious to see whether or not Bf4 was a "Book Move", I went to the analysis mode and undid Nc3, and played Bf4, but it still said it was the Slav Defense: Three Knights Variation. I went to the explorer to check this, and it showed that it isn't a book move at all, (which is true). So...yeah.