From what I gathered. In terms of CoA. There is Koka (Hardening). Emission (Barrier). Internal Destruction. Koka is the most common and available to most fighters. Emission has been utilized by Admirals, Kuja, Rayleigh and Scabbards. Its the haki used to "coat" projectile attacks. Internal Damage as per Kaido's confrontations have all been limited to CoC wielders - Roger, Rox, WB, Oden, Shanks and now Luffy. This means other Conquerors are liable to use the technique (Ray is proof). Furthermore, Internal Destruction is only achievable through combining Emission and Conquerors. As it stands Kaido can be injured with all variants but his fruit auto-heals. However, due to the nature of Internal Damage, his wounds are more permanent. It explains how Oden wounded him. And why even though the scabbards came close, all feel short. None of them had it in them. All the more reason Kaido believes Zoro possess CoC. As nothing short of that would permanently wound him. As another CoC user Kid needs to show up and out against Linlin. Luffy is asking Zoro and Law to head back. Kid is running around like the trashman.