The Author's Understanding of Armament Haki

The Levels and Techniques of Armament Haki in This Book

(Note that even when using low-level techniques, those who have reached a higher level will achieve the same effect as those who have reached a lower level. For example, when someone at the Emission level uses Hardening, their Hardening will be harder than that of someone at the Hardening level.)

Coating: The Haki energy emanates from the body like invisible smoke, which can be used to enhance the attack power of limbs and weapons, and will not conflict with the elementalization of the Logia, but the emitted energy will gradually dissipate into the air.

Evidence: In Sabaody Luffy punched the Pacifista, he clearly said the word "Coating," and although his fist didn't turn black, the power was still considerable.

Hardening: Controls the Haki energy emanating from the body surface to condense on the body surface, increasing the body and weapon attack power while increasing its hardness and physical defense. It can also counteract various Devil Fruit abilities, but it conflicts with the elementalization of Logia Fruits, so the user cannot elementalize the Hardening parts.

Evidence: Smoker Hardening his fist, so he could only elementalize the part of his arm, which is why the Three Admirals basically didn't need to Hardening.

Emission: Further enhances control over Haki, releasing Haki energy outside the body for ranged attacks or defense. Proficient users can even condense an invisible armor around their body to block all attacks, but this technique consumes a significant amount of energy.

Evidence: The Three Admirals blocking Whitebeard's shockwave from a distance during Marineford used an Emission technique, and Sentomaru to send people flying in the Sabaody Archipelago was also this technique.

At the same time, the author personally believes that the Emission is what Wano Country generally refers to as Ryuo, Armament Haki flowing outside the body. This level is considered a high level of Armament Haki, and it is enough to hurt Kaido.

Internal Power: A higher level of Ryuo, it possesses the ability to inject Haki energy into the target's interior to cause destruction, building upon the Emission of Haki. This is a technique Rayleigh taught Luffy.

Evidence: Hyogoro explicitly stated that Luffy's technique is superior to the Ryuo he taught him, therefore it can be concluded that the Nine Red Scabbards Ryuo does not have the function of internal destruction.

(In summary: Ryuo, or Emission of Oden's power, can indeed injure Kaido, but the Internal Power Rayleigh taught Luffy can cause even greater damage. What kind of Ryuo Oden is still unknown. It's important to understand that when Kaido fought Oden, Kaido didn't yet possess top-tier strength. His daughter, Yamato, explicitly stated in the manga that after killing Oden, Kaido frequently left Wano Country to challenge others. It was during this time that he was constantly captured, executed, and escaped. Therefore, Kaido was still in his developmental stage at that time. Whether Oden, who could only injure Kaido, possessed top-tier strength is something that needs further consideration.)

(These are my thoughts. Feel free to share your opinions if you have any. After all, there's no official explanation yet, so everyone is just speculating and interpreting. This can be considered a secondary setting for the book, which is fine. It's just to give readers a clearer understanding of the different levels of Armament Haki in the text.)