The bonito is filleted into three parts or more, and basically it's only after these fillets are cut in 'fushi' (the shape of a wooden ship) and processed that the fillets can be called katsuobushi.
基本的には魚体を三枚以上におろし、「節」（ふし）と呼ばれる舟方に整形してから加工された物を指して鰹節と言う。