Puff is usually plastisol with a foaming additive stirred in, though water-based puff bases exist too. It prints flat like normal ink, then swells in the dryer as the additive expands. The amount of additive, the thickness of the deposit and the dryer temperature all affect how high it rises and how rounded the edges look; too little heat leaves it flat, while too much can make it collapse or crack.
Because the ink grows, it softens detail. Thin lines and small text spread and fill in, so puff suits bold lettering, chunky logos and accents rather than intricate art. It is printed through coarse mesh for a thick deposit, and puffed colors often look a little lighter and more matte than the same color printed flat.
Ask to see a sample or a strike-off at the height you expect. Puff ink looks different from both high-density ink and 3D puff embroidery; the puff print guide compares them.