What is the reference pressure in the buoyancy equition?
 

the solver theory guide says, when the buoyancy is activated, the absolute pressure will be caculated as followed:

pabs=p+pref+rho_ref*g*(r-rref)

with rho_ref the reference density, g, r and rref as vectors.

Is the pref in the equition the reference pressure, which is setted in the basic setting of the domain setting or is it the pressure at buoyancy reference location?

Yes, pref is the reference pressure of the domain.