Oda has confirmed over and over again, that there is a genetic component to Haki. Some people are more genetically inclined towards certain types of Haki. Like Rayleigh said, even "within the realm of CoO" Luffy is predisposed to sense emotions. I think this is why Luffy is a CoC specialist, Zoro a CoA specialist, and Sanji CoO specialist. It's all according to their personalities:
CoC - Kings/Leaders (Usually pirate captains or leaders)
CoA - Human weapons (Little emotion from them. Pure fighters like Zoro/Mihawk. Mihawk doesn't even have a crew)
CoO - Empathetic (Caretakers like Sanji/Katakuri. SHs often become "Twirly Hat" crew. Katakuri was the primary caretaker of BMPs)
But, we've seen with Katakuri and Don Chinjao, that they might very well be specialists in another set of Haki, even though they possess CoC. Chinjao was renown for his "drill" and that was primarily a function of his CoA. It is even mentioned in the magazines Garp surpassing Chinjao was because he trained his Haki to a high enough level. While Katakuri's most notable Haki is his observation, which again leads to the fact that he is a CoO specialist.
I think whichever Haki you unlock FIRST is the one you specialize in. Like Nen in HxH, that's just the easiest. However, you can still get stronger in other categories.
CoC specialists: Luffy, Ace, Doflamingo, (WB/BB are also obvious ones but we need to see more) -> Kings/Leaders
CoO specialists: Coby, Sanji, Otohime, Katakuri, Fujitora, Usopp -> Empathetic/emotional characters
CoA specialists: Zoro, Mihawk, Vergo -> Human weapons with little emotion
Zoro/Sanji are not specialists in CoC, but that doesn't mean they can't eventually unlock CoC. Remember 2 years ago they had no Haki at all, and they're still young. Sanji at the very least is a prince and of royal bloodline, so he has potential for CoC.
Sanji's case at the very least is very like Katakuri's. When Luffy (or Zoro) is/are away, he seems to be the primary caretaker of the Straw Hats just like Katakuri. They are CoO specialists of royal bloodline.
CoC - Kings/Leaders (Usually pirate captains or leaders)
CoA - Human weapons (Little emotion from them. Pure fighters like Zoro/Mihawk. Mihawk doesn't even have a crew)
CoO - Empathetic (Caretakers like Sanji/Katakuri. SHs often become "Twirly Hat" crew. Katakuri was the primary caretaker of BMPs)
But, we've seen with Katakuri and Don Chinjao, that they might very well be specialists in another set of Haki, even though they possess CoC. Chinjao was renown for his "drill" and that was primarily a function of his CoA. It is even mentioned in the magazines Garp surpassing Chinjao was because he trained his Haki to a high enough level. While Katakuri's most notable Haki is his observation, which again leads to the fact that he is a CoO specialist.
I think whichever Haki you unlock FIRST is the one you specialize in. Like Nen in HxH, that's just the easiest. However, you can still get stronger in other categories.
CoC specialists: Luffy, Ace, Doflamingo, (WB/BB are also obvious ones but we need to see more) -> Kings/Leaders
CoO specialists: Coby, Sanji, Otohime, Katakuri, Fujitora, Usopp -> Empathetic/emotional characters
CoA specialists: Zoro, Mihawk, Vergo -> Human weapons with little emotion
Zoro/Sanji are not specialists in CoC, but that doesn't mean they can't eventually unlock CoC. Remember 2 years ago they had no Haki at all, and they're still young. Sanji at the very least is a prince and of royal bloodline, so he has potential for CoC.
Sanji's case at the very least is very like Katakuri's. When Luffy (or Zoro) is/are away, he seems to be the primary caretaker of the Straw Hats just like Katakuri. They are CoO specialists of royal bloodline.
But I doubt both of them will unlock CoC in Wano right now to be honest:cheers: