With the fluid medium, the subsequent color-pouring mixture is then only thinner and you have a lower layer thickness on the canvas. Compared with the previous Pouring Medium, the fluid mixture flows faster over a canvas and then dries faster (because less layer thickness).
For the inexperienced, the Fluid medium is easier to handle when it comes to cell formation, because with the low-viscosity medium, the cells formed "blur" when it is moved too quickly back and forth.