Maybe that's the part of the haki a DF consumes when someone eats it.
It would explain how Roger could keep up with WB and his Gura while both seemingly had the same Haki setup.
It would also explain why Shanks hasn't weakened much after losing his arm.
And why Mihawk DFless sits at the top of the swordsmen.
And why Kaido said Haki transcends it all but then proceeded to lose against a DF. Haki may transcends all but not his haki as he can't fully use adv CoC due to his DF.
A lot of headcanon obviously. For now.