Dot gain is the amount that an ink dot spreads out - after it hits the substrate.
Because, the dot gain can change radically when you change your vinyl supplier. That's because the dots are too small, and too much of the white background shows thru between the dots. If, on the other hand, the cheaper vinyl has too much dot gain, the colors will bleed together and the sharp edges will look terrible. Generally, vinyl tends to have too little dot gain - while paper tends to have too much dot gain. It's easy to see why.