Digital BW, The Print

I assume you mean the ABW (Advanced B&W Photo) mode. If this is the
case, the answer is: when color toning is disabled (e.g., Horizontal
and Vertical are set to zero), the printer uses K (either MK or PK),
LK, LLK, LC, LM, and Y. The inks not used are C and M. C and M may be
used if color toning is enabled.