To explain this, we need to first explain the idea of the verse, “שְמַע יִשְרָאֵל ה' אֱלֹקֵינוּ ה' אֶחָד-Hear Yisrael, Hashem who is our G-d, Hashem is One.”
The meaning is: After the worlds were created it is possible to say that Hashem is “אֶחָד-One,” since the word “אֶחָד” means that Hashem is the א-One Master over the ח-8 aspects of the 7 heavens and the 1 earth, and ד-the four directions of the world, north, south, east, west.
So too above, in the spiritual realms, after Hashem’s Attributes were revealed from Him, it is possible to say that Hashem is “אֶחָד-One” over them; He is א-One master over the ח-8 aspects of the 7 Attributes of “1-Greatness, 2-Might, 3-Splendor, 4-Victory, 5-Majesty, 6-for all on heaven and earth is Yours, 7-to You Hashem is the Kingship, and (the 8th aspect is that) You are exalted, supreme over all rulers,” and the ד- four aspects of the angels described as “animals” that have “four faces” described in the verse, (Yechezkal 1:10) “The face of a 1-lion on the right side of all four angels; and the face of an 2-ox on the left side of all four angels; and the face of an 3-eagle for all four angels; and the main face they had was 4-the face of a man etc.”
We see from all of the above that one level of Hashem’s Oneness-אחד is reflected in how He is the One Master-א of the 8 aspects-ח and 4 directions-ד of the physical world. On higher level, He is the One Master-א over the 8 spiritual levels of His Attributes in Atzilus-ח and the four aspects (‘faces’) of the angels-ד.
And “He unites all of them” (Zohar II 114b), since of all creation are nullified to Him, so through all of creation being nullified to the One Creator, also all of creation becomes unified under Him.
However, compared to Hashem’s true Essence, the way He is before the worlds were created, we cannot in any way refer to Hashem as “אֶחָד-the One Master of all worlds,” since He is completely beyond the entire category of ‘worlds.’
