Jujutsu Kaisen has several elements that were left either unexplained or underdeveloped, with the Special Grade designation perhaps being one of the most problematic in both categories. That is because there is the character of Masamichi Yaga, not Satoru Gojo or Yuta Okkotsu, who shows the problems with this designation.
It is confirmed in the Jujutsu Kaisen manga that a Special Grade sorcerer is someone with the power to overthrow a nation, which makes sense with the power of Gojo or Yuta.
However, Yaga's abilities made him a candidate for the Special Grade designation. This has led to discussions regarding the morality of the higher-ups since they are revealed to be the ones who control Japan, adding another unexplored layer to his discussion.
Disclaimer: This article is a speculative theory and reflects the writer's opinion.
The character of Yaga highlights the flaws of the Special Grade category in Jujutsu Kaisen

It was established in the manga that a Special Grade sorcerer earns that status if he or she has the power to take over an entire nation.
However, the case of Masamichi Yaga reveals that his ability to create Cursed Corpses who could have Cursed Energy and sentient nature, such as Panda, has the element that the higher-ups were afraid he could overthrow them, revealing at some point in the story that they control Japan as well.
Therefore, it is safe to theorize that the higher-ups created the Special Grade category as a way to tag those sorcerers who could be a problem for their authority, as evidenced by the different times Satoru Gojo challenged them.
However, this also presents the problem regarding why most sorcerers listen to them because, at least based on what was shown in the manga, the higher-ups don't have any powers.
The fact that Gojo managed to kill them before the events of the final battle with Ryomen Sukuna highlights how he always could have done that, which he mentioned early in the series.
That is consistent but the fact this didn't happen before is also quite bizarre, especially regarding the nature of the major clans.
Why this needed to be developed

A major aspect of the early stages of Jujutsu Kaisen was how the society of sorcerers was corrupt and how Gojo wanted to change that through teaching, with students such as Yuta Okkotsu and Yuji Itadori representing said change.
However, this idea seems to be thrown out of the window when he decides to kill the higher-ups, which is something he claims would be useless in the long run.
Some may argue that this was done in self-defense since they wanted to execute Yuji for something beyond his control and take Yaga's life for something he didn't deserve.
That is understandable but the question remains why no one else did this before, especially regarding how they don't have a Binding Vow or anything of the sort that could hinder them.
Final thoughts
The concept of Special Grade has always been somewhat obscure as a category in Jujutsu Kaisen but the idea highlighting the corruption of the higher-ups could have been explored a lot more.
Therefore, it is one of the many concepts of the series that should have been developed.
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