The question is to find the shear stress at the points A, B and C. The solution uses the sectioning planes aa, bb and cc to do so.
For example, at C, how should I know whether to use a horizontal or vertical sectioning plane. Also, how should I know which portion of the beam i would have to consider as A', the bit to the left or right of the sectioning plane ?
Shear Stress(Flange) = q(Flange)/thickness(Flange)
where q(flange) = VQ(Flange)/I
Shear Stress(Web) = q(Web)/thickness(Web)
where q(Web) = VQ(Web)/I
V is the transverse shear force
Q = A'y" is the first area moments of the area A' with respect to the neutral axis