It is only this one single cheeseburger which it does not contain. The Cheesebox contains all other cheeseburgers, all other burgers, all other parallel versions of this cheeseburger in other universes, all exact or inexact copies of this cheeseburger found elsewhere in The Box, the concept of cheeseburgers, the concept of that specific cheeseburger, the concept of the cheeseburger being in the Cheesebox, the concept of it not, the ingredients, the concept of the ingredients, the particles that make up the cheeseburger, all of the above are within Cheesebox. But the cheeseburger itself is not. It is the only thing separating Cheesebox from being a Box-equivalent structure. (As in, a structure containing the exact same things as the Box.)
Cheesebox is incredibly important as it is often assumed that containers like The Box and entities like The True God cannot be challenged without being equivalent to them. For example, it is often assumed that an Omnipotent entity is infinitely more powerful than all non-Omnipotent entities. But Cheesebox disproves this. Cheesebox is clearly inferior yet still very much comparable to The Box. Thus, you could equally have an entity weaker than The True God who is still capable of challenging the power of The True God.
Many may say that the existence of the Cheesebox is impossible as containing everything in the Box with the exception of a single cheeseburger would mean it contained many objects that would force it to contain the cheeseburger and thus become equal to The Box. Alternatively, it could still not contain the cheeseburger and instead would be forced to not contain many other objects. This is one of an uncountable number of ways in which the Cheesebox would be prevented from containing everything except a single cheeseburger. Clearly, containing 'almost as much as the Box' is completely impossible.
However, we must not forget that it is equally impossible to contain everything. Just as there are objects in The Box that force the Cheesebox to contain the one cheeseburger outside of it, there are also objects that cannot be contained, or objects that prevent its containers from containing everything, thus making it impossible to contain everything at once. Yet The Box still does so anyway. This is because The Box has what is known as Maximal Paradox Resistance. (Often shortened to MPR.) MPR allows an object to ignore any paradoxes or impossibilities that arrive from performing an action and performs the action anyway. Thus, a container with MPR can contain an object that cannot be contained, as well as containing everything including objects that prevent their containers from containing everything. Like The Box, Cheesebox also has MPR, and thus is able to successfully contain everything except one single cheeseburger. Though this is obviously impossible, MPR allows the Cheesebox to ignore everything making it impossible and do it anyway.
This means the Cheesebox truly does contain almost as much as the Box, unlike almost all other structures which contain negligible fractions of The Box. Structures such as the Cheesebox are known as Near-Maximal.
The implications of an object being able to contain almost as much as the Box are immense. It is normally assumed that maximal objects and entities such as The Box and The True God cannot be challenged - they can have no equals and nothing weaker than them that can nonetheless compare to them. A core part of Powerlisting is that Omnipotence in unchallengable, which was also a core part of power scaling for a long time. As large as objects like the Omniverse, Barrelplex, Schemafield and Itfield are, they are so small compared to the Box that they are closer in size to the Universe than to The Box. It is often believed that no object can be comparable to the Box in size without being equal to it, yet Cheesebox disproves it. This means many different objects comparable in size to the Box can exist, for example an object that contains exactly half as many objects as the Box contains, and many more. These are known as 'Near-Maximal- objects.
But the impact is even broader than that. If this is applied to the power of Cosmic Entities, we can say that just as Near-Maximal containers can contain almost as much as The Box, Near-Maximal entities can be almost as powerful as The True God. Challenging Omnipotence is often thought to be impossible, yet clearly it must be possible, as Near-Maximal entities would be capable of doing so. For example, if Cheesebox contains everything but a single cheeseburger, a hypothetical "True Cheesegod" would be capable of doing everything except making a decent cheeseburger. Some may say that this would mean he has very few skills as many abilities may require one to be able to make a decent cheeseburger, but as The True Cheesegod has MPR, he will be capable of performing these abilities without making a decent cheeseburger. As such, he could be a serious threat to The True God despite not being Truly Omnipotent and though he would likely lose in a fight, it would be an incredibly close fight as ones ability to make a decent cheeseburger isn't exactly a deciding factor in most battles.
Beyond just structures and entities, you could also have Near-Maximal numbers, Near-Maximal transport, polygons with a Near-Maximal number of sides, and even food that is Near-Maximally tasty. Anything that can have a maximal variant can have Near-Maximal equivalents.
While the rest of this article has been focusing on the contents of the Cheesebox, this section will focus on the only thing it doesn't contain; the single cheeseburger. Some may suspect that this cheeseburger has some impressive properties. Surely, if it is not contained by the 2nd largest structure currently detailed on the Wiki, it must be impressive? But this is not the case. The cheeseburger existed within our own universe, on our own planet. Statistically, it's likely that the cheeseburger was produced and served at a McDonald's restaurant in the USA, and eaten by an ordinary human, with no further effect.
So how can something as large and powerful as the Cheesebox have no effect on something so small and normal? Well, The Box and the Cheesebox are not strictly -verses, they are Fields, meaning them containing certain objects has no effect on these objects. But then why does such a specific field exist, given how ordinary the cheeseburger is? As The Box contains everything, it must thus contain a field that contains everything except one single cheeseburger, as if it did not contain such a field, it would not contain everything. Thus, we can guarantee that the Cheesebox exists somewhere within the Box.