Black knockout.

Removing black and near-black from the artwork before it prints on a black garment, so the shirt itself becomes the black and no ink or white goes down there. In CADlink Digital Factory it is a filter, KnockMeBlackOut, with a Clean black option that turns near-black to black first so more area drops out; the tool page adds a tolerance slider that sets the cut point between what prints and what turns transparent. Knockout is not a reason to strip white from the dark tones that stay in the print: CADlink's underbase strength page says most DTG needs some white under black areas for the shirt to wash properly, with presets that run from 100% white under black down to 0%. It is a print shop setting, not something the seller switches on, and the seller's only lever is leaving the area transparent in the file. Shops get it wrong at both ends. Skip it and black on black prints over a full underbase, which Printify says can come out grayish or dull; drop the tolerance too low and dark shadows and navy outlines vanish into the shirt, a fault nobody sees until the garment is off the dryer.