The Mishna says Kol Shelo Ba B'Chamin MeiErev Shabbos Medichin BiChamin BiShabbos. Query: If it is referring to Kalei Bishul even Irui Kli Shenei is not allowed and if it is referring to non-Kalei Bishul even to cook inside a Kli Sheini itself (Shoreh) should be allowed, so why single out Medichin which implies that only Irui is allowed?
TOSFOS (39a, DH Kol she'Ba) asks your question. Tosfos cites the RI who answers that even though a Kli Sheni does not cook, nevertheless we may not be Shoreh a food in a Kli Sheni because it *looks* like one is cooking. We may only place spices into a Kli Sheni because that does not look like the act of cooking.
Alternatively, suggests Tosfos, it is permitted even to be Shoreh in a Kli Sheni. The Mishnah mention Madichin only to teach that when it comes to the Kulyas ha'Aspanin fish, even Madichin is forbidden.
Yours, Mordecai