Ink channel.

One ink's dedicated path through the press, from its supply pack through the line and damper to the nozzles assigned to it. The count fixes your color set, and on machines that can double a color up it moves throughput too: a press plumbed for CMYK plus white has every channel spoken for, so a spot color, a fluorescent or a second white has to come out of an ink already loaded. White is the channel that behaves differently, because its pigment is dense and settles out of suspension, which is why spec sheets that list an ink circulation function list it for the white channel. Converting a channel to another ink means a flush and a purge, which costs ink and stops the press, so it is not a decision you make between two orders.