Alpha channel.

The channel in an image file that stores how opaque each pixel is, from fully transparent to fully solid. On DTG and DTF it is the instruction for where white ink goes, because the RIP builds the underbase or the film's white layer from it. A JPEG has none, and Printify warns that it forces a solid background, usually white, which prints as a box round the design on a dark shirt. The values between empty and solid are where shops lose money: Printify describes a 30% transparent pixel getting a 100% solid white dot underneath it, which turns a soft glow into a chalky halo. RIPs do not agree on this, since CADlink offers an Adaptive mode that scales white density by opacity, a Valid Pixel method and a switch that treats every semi-transparent pixel as opaque, and a seller cannot see which one the shop runs. On a garment the safe file has exactly two kinds of pixel, and a shadow that has to fade is built as a halftone. Wall prints want the opposite, because they are full coverage: SAIC asks for files flattened with no alpha channels, Gelato recommends flattening all transparencies, and Printful names JPEG for full-coverage posters and canvas.