high-grade katsuobushi (mold-cultured dried bonito)
Refers to katsuobushi that has been further dried and fermented with mold (usually Aspergillus glaucus), giving it a deeper, less fishy flavor. Used in high-end dashi.
High-end Japanese restaurants often use ichiban-dashi made from karebushi.
Karebushi is katsuobushi that has been aged with mold.