The SBS answer is still relevant. A specialty just means you have the best mastery of that Haki type compared to the other types that you have.
Luffy is most the proficient at using CoC than CoO or COA.
Zoro is most the proficient at using CoA than CoC or CoO.
Sanji is most the proficient at using CoO than CoA or CoC.
Katakuri's CoO mastery is much better than his CoA or CoC.
Note: I deliberately did not say the strongest because you can't really directly compare all 3 types of Haki in terms of power. And CoC is usually the strongest type even when you're not that proficient—Zoro's CoC is stronger than his CoA even though Zoro's better at using CoA.
It doesn't mean, however, that your specialty is stronger/better than someone who doesn't have it as a specialty. You may or may not have a better mastery than others even if it's your specialty.