What is a Zeres?
Rashi: A Zeres is half an Amah. 1
The distance between the small finger (Zeres) and the thumb that are both extended to their maximum.
What exactly is the Torah saying here?
Rashi: The Torah is saying that the Choshen, which initially measured one Amah by half an Amah, was folded, so that ultimately, it measured half an Amah by half an Amah, with room in its folds (like a pocket. to house something 1 [Rashbam]).
See Pasuk 30.
What purpose did the pocket in the Choshen serve?
Rashi and Ramban (Pasuk 30) and Rashbam: It housed the Urim ve'Tumim. 1
Which will be explained later in Pasuk 30.
Why does the Torah not mention them here?
Ramban #1 (on Pasuk 30): The Torah deliberately curtails mention of them due to their mystical connotations. 1
Ramban #2: The Torah only mentions the Urim ve'Tumim when instructing Moshe to place them inside the Choshen. It does not mrntion them earlier, because, neither the people, nor the craftsmen 2 had a hand in them - neither as regards the donation nor as regards making them. In fact, they were made either by Moshe in sanctity, based on a secret formula of which only Moshe was aware, or by Hashem Himself. 3
Seeing as four letters ('Ches', 'Tes', 'Kuf' and 'Tzadik' do not appear in the names of the twelve tribes, how could the Urim ve'Tumim be guaranteed to answer all the questions that were put to it?
Ba'al ha'Turim (Pasuk 17): The words 'Avraham, Yitzchak, Ya'akov Shivtei Yeshurun' were added to the names of the tribes. 1
Arranged in the following way, so that each stones contained six letters, making a total of seventy-two (Refer to 28:30:1:1*): Reuven - 'Alef'; Shimon - 'Beis; Levi - 'Resh' 'Hey' 'Mem'; Yehudah - 'Yud'; Yisachar - 'Tzadik'; Zevulun - 'Ches'; Dan - 'Kuf' 'Yud' 'Ayin' Kuf'; Naftali - 'Beis; Gad -'Shivtei'; Asher - 'Yud' 'Shiyn' 'Resh'; Yosef - 'Vav'; Binyamin;
In which order were the names of the tribes engraved on the stones.