what counts as a book move?

I play the Rousseau gambit (f5 against the Italian) which is a legit opening and for some reason it is evaluated as a mistake rather than book move by the chess.com engine, even though you are only sacking a pawn (and it's not really even a sac because white is supposed to play d4 in response to get the best position rather than exf5 (that is already counted as a mistake))

yet there are some gambits where you sac an entire piece or 2 even (Muzio gambit) and they are in the book opening category

The requirements for most book moves here are that they are played since minimum number of times by mater level players, though I don't know the specific amount of games to qualify.

A Book Move is a good move that is in the Opening and Master-Approved. Most masters disapprove of the Rousseau Gambit as it wastes the most important tempo in the Italian.

damn masters not knowing how to play chess lol