Siman 50

The reason they established the chapter "Eizehu Mekoman" and the Baraita of Rabbi Ishmael after the section of the Tamid and the Ketoret is because we say in Kiddushin (30a) that a person must study every day Mikra, Mishnah, and Gemara. There is much Mikra in the prayer, and for Mishnah they established "Eizehu Mekoman" because it is written: "And in every place incense is burned and offered to My name"; but is it so that they offer incense in every place? Rather, these are the Torah scholars who engage in the laws of the Temple service in every place—I credit them as if they are sacrificing and offering to My name (Menachot 110a). In this chapter, all the sacrifices are included, and it is also without dispute. And the Baraita of the thirteen attributes was established because these are the essential principles of the Oral Torah. It is taught at the beginning of "Torat Kohanim" at the start of all the sacrifices. The Midrash of the Torah is like Gemara, and therefore they established it after "Eizehu Mekoman." One must understand what he studies, for if not, this is not study. It has already been explained in siman 1 that regarding an Olah, Todah, and Shelamim, one should say "May it be Your will... as if I sacrificed," see there.

There is one who says that in a house of mourning one does not say this, because it is study and a mourner is forbidden from Torah study. But it is not so, for anything that is the order of the day is permitted. And since they established them in the order of the day, it is like the entire prayer, as I wrote in Yoreh Deah siman 384.

And know that the prayer until after the Shemoneh Esrei is divided into four divisions: the first until "Baruch She'amar," the second until "Yotzer Or" and these are the Pesukei D'zimra, the third from "Yotzer Or" until "Ga'al Yisrael," and the fourth is the Shemoneh Esrei. And they are hinted at for high and hidden matters. And there is a difference between Nusach Ashkenaz and Nusach Sefarad: for in Nusach Ashkenaz, after the Baraita of Rabbi Ishmael, they say "Mizmor Shir Chanukat" and afterwards "Baruch She'amar" and afterwards "Hodu" until "Romemu" and additional gathered verses, such as "Vehu Rachum," "Hashem Tzeva'ot," and "Mizmor L'todah." And afterwards eighteen gathered verses from "Yehi Chavod" until "Ashrei," and "Ashrei" until the end of Pesukei D'zimra. And it turns out that "Hodu" and all the verses are also included in Pesukei D'zimra, for they are also words of praise. And the Sefardim say all this except for "Mizmor L'todah" before "Baruch She'amar," for they hold that the essence of Pesukei D'zimra is from "Tehillah L'David." And they add further the verses of "Hashem Melech..." and "Lamnatze'ach Binginot." And these and those are the words of the living God, and every place is according to its custom. And a person should not change because of dispute, and also it is like "do not cut yourselves."

And the reason for saying these verses: for "Mizmor Shir Chanukat," Solomon said it at the time he brought the Ark into the Holy of Holies, and he was not answered until he mentioned the merit of David, as is found in Shabbat (30a). And since all of Pesukei D'Zimra are the verses of King David, therefore they preceded them first with this psalm in which his righteousness was revealed. And therefore they did not include it within Pesukei D'Zimra (it seems to me). And the verses of "Hodu L'Hashem Kiru Vishmo," it is found in Seder Olam that all the years the Ark was in the curtain, David established to say this psalm. And the verses of "Hashem Tzevaot," we say in the Yerushalmi, chapter "Ein Omdin," that these verses should never move from your mouth, see there. And the eighteen verses of "Yehi Khavod" correspond to the eighteen blessings of the Shemoneh Esrei. And therefore, even according to the custom of the Sephardim, they included them within Pesukei D'Zimra.