The Talmud is, of course, the great reservoir to which R. Isserles turns as the first step in attempting to solve a problem. The question at hand is immediately referred to an identical or similar case in the Talmud. The second step is the weighing of the opinions of the ראשונים, i.e. Alfasi (רי”ף), Tosafists, Nachmanides, etc. expanding and explaining the text. The opinion of the majority is followed by R. Isserles and even Maimonides, whom he respected very highly, is disregarded if he was in the minority. After the Risonim, R. Isserles proceeds to examine writings of אחרונים, i.e. Mordechai, Ashri and Tur, and the latter is followed especially when the Tosafists agree with him. At this point, the Responsa of still later authorities are cited extensively in accordance with the well-established priniciple of הלכה כבתרא, paying due attention even to the opinions of contemporaries and to customs of Polish Jewry which the ב”י omitted. Thus, R. Isserles, in his Responsa as well as in the ד”מ and his commentary on the Shulchan Aruch, served as a supplement and offered his community the code of Law adjusted to its authorities, customs, and needs. He spread the “cloth” over the table prepared by his contemporary, the ב”י.
Rabbi Asher Siev, “The Period, Life and Work of Rabbi Moses Isserles” (PhD. Diss., Yeshiva University, 1943), 57-58.