Semi-transparent pixels.

Pixels whose opacity sits between empty and solid: anti-aliased edges, soft shadows, glows, fades, a low-opacity texture. A screen blends them into the background; a white-ink printer has to decide whether to put white under them. Printify's blog says any value above 0% gets solid underbase, and a 30% transparent pixel gets a 100% solid white dot underneath, which is the chalky ring round a design on a dark shirt. RIPs do not agree: CADlink Digital Factory offers an Adaptive mode that scales white by opacity as well as a switch that treats every semi-transparent pixel as opaque, and Inkthreadable calls the results in between usually unpredictable. The seller cannot see which one the shop runs, so the safe garment file has none: hard edges, or a halftone where a fade is needed. Sublimation is the exception, since there is no white ink to put under them, and Redbubble rates fades to nothing as OK there.