At Shalosh Seudos - the third Shabbos meal often the song B'Vo'o M'Edom/Yetzave Tzur Chasdo is sung. Some people/Shuls start from B'Vo'o M'Edom and some people/shuls start from Yetzave Tzur Chasdo. What is the source of these different Minhagim? Also why do we not sing the entire song from the beginning Boruch Hashem Yom Yom?
The commentary on the "An Invitation to Piyut" site observes (my translation) that "the section beginning Yetzaveh... containing utopian descriptions of the Redemption, found its place in the atmosphere of yearning and longing that pervades Seudah Shelishis"; much the same could be said about Bevo'o me-Edom.
So I don't know the actual source for dividing the zemer this way, but it does make sense, since each of these points does mark a distinct shift in the zemer, in contrast to the first six stanzas which describe how Hashem has sustained us during each of our exiles.