Photo Black and Matte Black
Two distinct black inks formulated for glossy and matte papers respectively, only one of which can be active at a time on many printers.
In depth
Photo black is designed to sit on a glossy coating and achieve a deep maximum density; matte black is formulated to penetrate uncoated fibres, where photo black would look grey and weak. Switching between them purges the line and wastes ink, which is why many print studios batch jobs by paper type.
Examples in practice
Switching from matte to photo black costs several millilitres of ink, so glossy jobs are grouped together.