Their book provides the answer.
Maintaining and Transporting the Printer | 111
Changing the Black Ink Cartridge
You can use either Matte Black or Photo Black for printing. Both cartridges
are installed all the time, and the printer knows which cartridge to use
depending on the Media Type selected in the printer driver. You may
sometimes need to switch cartridges as described below when using
non-Epson media.
The Photo Black cartridge is ideal for printing photographic images on all
media types. It is optimized for traditional photographic and ink jet-coated
papers, such as Epson Ultra Premium Photo Paper Luster and Epson Photo
Paper Semi-gloss, etc.
The Matte Black cartridge is optimized for printing photographic images on
matte or plain papers, such as Epson Ultra Premium Presentation Paper
Matte and Epson Velvet Fine Art Paper.
Generally, you should switch black ink cartridges only if you plan to run a
large number of print jobs in the new mode. Since it takes several minutes to
switch and consumes some ink, you shouldn’t switch cartridges more often
than necessary.
111 is the page number.
Rich