Guys this is just my opinion not manga confirmed facts but from my understanding, flow armament haki is not advanced CoA but just another, more efficient way to use CoA the other being hardening. In my opinion using CoA to coat your weapons, in conjunction with the DF(flame patterned CoA), flow armament and hardening(plus super hardening used by Katakuri and Jinbei) are just different techniques and applications of CoA. The only technique that I consider as advanced CoA is the internal destruction only used by Luffy and Rayleigh so far.
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Exactly, the concept of flow is a general haki thing that can be applied to all 3 types of haki it is not advanced CoA or CoC