Thomson scattering on the electrons does not produce any circular polarization?

Question

All references on CMB polarization has this statement as if it is a trivial fact. But I have to admit that I completely don't understand what this sentence is telling us.

Answer 1

Yes, it is a consequence of the formulation of the Stokes parameters of the outgoing beam of a monochromatic unpolarized incident plane wave and can be generalised.

By symmetry, Thomson scattering can generate no circular polarization, so V = 0 always and will not be considered further.

Introduction to microwave background polarization chap 3 , Polarization from thomson scattering