Dwell time.

How long a piece is actually held at heat, counted separately from the number on the temperature controller. Time and temperature are separate conditions, which is why AATCC TM133, the colorfastness test for hot pressing, specifies temperature, time and weight rather than temperature alone: a platen at the right temperature still has to drive that heat through the fabric and into the ink film. Thickness, moisture and a cold shop all slow that climb, so the same settings do not behave the same on a heavy hoodie as on a light tee, and the only honest check is a probe on the ink rather than the dial. Cut dwell short and the ink sets at the surface only, then fails in the wash weeks later and comes back as a complaint and a reprint on a batch you already invoiced.