In Chassidus, there are three basic components of spheroes, the specific expressions of Hashem in Seider Hishtalshelus:
1. Oir Ein Soif — Infinite Light of Hashem
2. Pnimiyus ha'Keili — the Essence of a Vessel
3. Chitzoinius ha'Keili — the outer aspect of the Vessel
It makes sense the the Vessels should be of a lower level than the Light. Light is infinite, simple and expresses Hashem. Vessels are finite, have specific properties or aspects (or, rather, because of them the specific aspects of Light that had been so far hidden in potential are expressed), and conceal and define/shape the Light. Vessels come from Hashem's Potential Not to Shine (Yecholto SheLoi L'hoir), while the Lights come from Hashem's Potential to Shine (Yecholto L'hoir).
On the other hand, if you look at the specific sub-divions of the Vessels, something interesting is revealed: the second level in the enumeration above, the Essence of the Vessels, is not only higher than the outer aspect of the Vessels; it's even higher than the Infinite Light itself!
How are we to understand this?
From Kabbalistic point of view, we can explain it this way. The essence of the vessels is the reason, the motivating force of their action, while their outer aspect is the action itself. The Infinite Light is an expression of Hashem. The ultimate desire of Hashem, however, is that His expression reaches its specific target in the Seider Hishtalshelus. For this reason, the Light must be limited and shaped.
Therefore, as we learn from Kabbala of Arizal and from hemshech Ayin Beis of Rebbe Rashab, the force that motivates the action of the Vessels is actually mushrash b'Atzmus — its source is in the Essence of Hashem! Therefore, it makes sense that it is higher than the Light; since Essence of Hashem is infinitely higher than His expression through the Light, the "motivating force" for the keilim that stems from it (and becomes pnimiyus ha'keilim) is also higher than the Light.
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To give an example from music, a melody consists of notes. Notes result from a string being divided — for instance, by pressing on it at a specific spot with a finger. Where exactly the string is divided determines what frequency it will vibrate at, which determines the note. If you only divide the string, however, no music will result. The string also has to start vibrating (through someone plucking it, or bowing, or striking it with a little hammer).
So, there are two aspects to making a note: the vibration of the string and the fact that it is divided and thus is vibrating at a particular frequency (pitch). [Of course, there is also how hard it is plucked, which determines the volume of the note, but we won't go there.]
So, division of the string is the keili, and its vibration is the oir.
But there is a third component: the one who divides the string. The purpose of the division. That is the pnimiyus ha'keili.
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One can also give an example from a relationship. Let's say, one buys flowers for his wife. Why does he buy them? Because he wants her to feel nice. So, the intention is the oir, and the flowers are the keili, the way of expressing that intention. But there is the third component: remembering to buy the specific flowers that the wife likes. That's pnimiyus ha'keili.
And perhaps we can say that remembering which flowers she likes (or what kind of specific favor she wants at the moment) is even higher than the whole act of buying flowers altogether.