In the Shabbat davening there is the phrase "v'yanuchu ____ yisra'el..." The middle word, according to the Artscroll text, is "bah" in ma'ariv, "bo" in shacharit and musaf and "bam" in mincha. This is explained with a reference to the Magen Avraham (O"C 268:3) that the word refers to 3 separate items: the sabbath of creation (the feminine form), the sabbath at Matan Torah (masculine) and the sabbath of the future (plural). I don't know if this system is in use in other siddurim so I am basing my question on this text.
On A Yom Kippur that coincides with Shabbat, this phrase is inserted in the amidah. The uses follow that pattern until Ne'ilah. On pages 716 and 754, the text is given as "bo."
Does this attest to something about the nature of Ne'ilah? Is it chosen for any particular reason?
