Yatta! I'm Uzumaki Kushina, your instructor today! And I'll be teaching you a bit about Fuuinjutsu theory Dattebane~!

The reason Fuuinjutsu is mostly a lost art and was focused on by the Uzumaki is mainly due to the fact that it is much like a written natural language. The semantics and syntax used in Fuuinjutsu differ as per user because everyone speaks and writes differently. They use their own phrases and sentence structure.

Syntax being: the way in which linguistic elements (as words) are put together to form constituents; and semantics being: the study of meaning; the historical and psychological study and the classification of changes in the signification of words or forms viewed as factors in linguistic development.

People, as a whole, speak differently now than they did in the Warring states era. People from different geographical areas speak differently as well because they have their own dialect. The reason the Uzumaki clan advanced in sealing arts like they did is because their population as a whole focused on it. The linguistic elements evolved in the passing of time and so, their sealing evolved with it. A single person focusing on their own Fuuinjutsu development gifting their designs to a foreign shinobi could and most likely would result in the foreign shinobi losing a limb when attempting to recreate their seal because the semantics they themselves use could have a completely different meaning behind it and so the intent behind their formula would result in disaster.

Because language is a learned thing passed from one person to the next, Fuuinjutsu is a constantly evolving language of balance and sometimes even completely recreated to suit the type of Fuuinjutsu for a single purpose. Which is mainly why it is taught from master to apprentice and why a lot of the same types of seals vary depending on the user, even if in the end they have the same result.

Each type within your main semantic language of Fuuinjutsu would have its own altered branch of written language. Fuuinjutsu is not just a language of calligraphy (although that is a highly important and fundamental part of Fuuinjutsu) it can also be performed through hand seals if one is proficient enough. Using hand seals to direct chakra through the tenketsu to perform a technique is normally something that takes time (and sometimes multiple people preforming the technique together) and so this method is normally not used in battle by anyone other than a sealing master unless the user has signed a summoning contract with one of the many different summons from the summoning realm. The extent of summons from the summoning realm is unknown due to its vastness.

Different types of sealing would be:

Container: A type of seal which "stores" something specific, such as a storage scroll. These scripts "Contain" whatever you put in them. The outer script is altered automatically depending on what it is placed on and what exactly it must contain. For example, when storing the body of a lost comrade, the outer portion of the seal (which is the blank center of the storage seal) alters itself to show the Kanji for "Body" once the sealing language has opened a small pocket dimension by using the small amount of chakra applied to the subjective surrounding Kanji making up the seal itself.

Barrier: A barrier is a balance of a placement of four separate sealing matrixes. When used alone nothing will come of it because the other matrixes are required to form the barrier. An exception of this would be the Uzumaki adamantine sealing chains which can create a complete chakra barrier on its own from a single point of origin. This exception is something that only an Uzumaki can do and the exact science behind this technique was lost in the fall of Uzushiogakure. An example of this would be the "Four Black Frogs Formation", which can seal a person or an object into a container temporarily, this specific sealing method can be reinforced by the "Dark Sealing Method" if whatever being barred from the outside world is too strong.

Sealing Method: Like the container, the sealing method is used to store something. Unlike the Containing method, however, the Sealing method is something meant to "Seal" something away either semi-permanently or completely permanent. An example of this seal would be the "Evil Sealing Method" which can suppress technique induced marks on a person and even some kekkei genkai.

Touch-Sealing: A method in which a single seal is applied to a surface by a single touch along with the application of chakra. This method is a combat sealing method and only used when the user has an intimate knowledge of the seal being applied. Otherwise you risk blowing your fingers off.

These explanations are only a few examples of the types of Fuuinjutsu that are possible. Listing all of them is an impossible feat because of how it is always evolving and changing.

There are a few basic rules when it comes to Fuuinjutsu. A seal must always be balanced. An odd numbered seal can be used to cause an imbalance in an even numbered seal and vice versa. Many seals are simple and are normally only two-tiered such as a basic explosive seal or a simple storage seal. The more complex the seal, the more tiers there are and the more room needed. Some seals, such as ones used on the human body, can be compressed or extended based on their complexity.

The master/apprentice relationship of Fuuinjutsu is a sacred one and the first thing the apprentice must learn is how to make their own sealing ink or just plain old calligraphy ink since calligraphy is such an integral part of Fuuinjutsu. Sealing ink can be made in many different ways. The most common method is by grinding the ink with water. More advanced methods use ink crafted from charcoal made by burning chakra conductive trees and grinding the ink with blood on an ink stone. However, when using blood to create ink, unless you have also used an anti-clotting agent in your ink; you can only create little at a time because of the clotting factor of the hemoglobin as well as the blood plasma. Due to this issue, it is best to have your ink prepared beforehand and sealed inside of a sealing scroll to keep it preserved in a small pocket dimension.

My favorite seals are storage seals ya know? Not only can they contain useful supplies, but I have somewhere to store all my ramen without all the clutter Dattebane!

Psst: Don't tell Mina-chan but I had to take some of his from his personal stash 'cause I ran out of sealing scrolls ya know?