Anti-aliasing.

The smoothing a design application applies to curves and diagonals by giving the border pixels in-between values, in colour or in opacity. On screen it removes jagged steps. On DTG and DTF it leaves a ring of semi-transparent pixels round every shape, and the RIP decides what happens to it: Printify says the machine reads any pixel that is not transparent as ink it needs to lay down, while CADlink has a Valid Pixel Tolerance option written to keep underbase off errant pixels such as anti-aliasing. Export over a coloured background and the ring carries that colour with it, which presses as a faint outline in the wrong colour. Switching smoothing off altogether gives stair-stepped curves and ragged small type instead, so the fix is a clean edge on a truly transparent background at full print size, checked in the alpha channel before the file goes to the queue.