ASBI Pocket Beit Midrash: Aruch HaShulchan, Orach Chaim 242:7

The Laws of Shabbat are vast and deep. And our rabbis, the Tur and Shulhan Arukh, for all of their length, wrote nothing more than what is forbidden and what is permitted, but they did not write the essence of the primary categories of forbidden labor (avot melachot) and their derivatives (toladot) and the violations of Shabbat by Torah law (d'oraita) and rabbinic law (d'rabbanan) and the general root principles of the categories of forbidden labor on Shabbat. And Rambam did explain everything. And for the sake of completing this holy endeavor, it was our intention as well to explain a little bit of this and the ways in which Rambam differs from our teachers Rashi, Tosafot, and other Rishonim as will be explained with God's help.