Sublimation.

Dye turns to gas under heat and penetrates polyester or a polymer coating, so the print sits “in the fabric”, or in the coating of a mug or panel. There is no white ink, so the blank has to be white or light, and it needs something for the dye to bond to: Sawgrass names 65/35 polycotton as the lowest blend for its Sublijet UHD ink, and an uncoated hard good comes out pale and patchy.