Did you read the manual?
http://manuals.ootpdevelopments.com/...tching_ratings
Relief pitchers receive a small bonus to stuff, to reflect the fact that batters get fewer looks at their pitches and have less of a chance to adjust to them. OOTP displays that bonus in the form of increased Stuff ratings. This bonus is heavily tied into the strength of a reliever's top two pitches, since relievers have much less incentive to mix in their weaker offerings. This means that a pitcher with a strong fastball, a strong slider, and a weak changeup might make for an average starting pitcher but a great reliever. You get to decide how he provides the most useful fit for your team. You can see the change in a pitcher's Stuff rating if you change the player's position from SP to MR and back.