CMB polarization is caused by Thomson scattering and subsequent gravitational lensing of CMB photons. CMB electromagnetic radiation (EMR) is mostly unpolarized, but somewhat under 10% is polarized. Polarization falls under the polarization modes, E-mode and B-mode (so-named in analogy to the divergence and curl of electric and magnetic fields as expressed in Maxwell's equations: not connected to the Maxwell equations or electricity and magnetism other than by analogy), with unpolarized EMR referred to as T-mode (T for temperature, as the EMR indicates black body spectrum associated with a specific temperature). They are also classified as primary modes or secondary modes, based on whether the polarization occurred at/before recombination, when the photons began their free streaming, or subsequent to it, such as from gravitational lensing. The primary modes included E-modes from Thomson scattering, from features in the density distribution due to initial fluctuations and baryon acoustic oscillations, and B-modes theorized to result from gravitational waves (primordial gravitational waveses, a theorized product of initial fluctuations). If the latter were found, they would be considered a confirmation of both gravitational waves and inflation cosmology theories. Secondary modes include B-modes created by gravitational lensing of the primary E-modes.
Angular power spectrums can be drawn by cross-correlating pairs of these, correlations known as BB, EE, TT, TE, TB, and EB. The latter two are always zero. TT is just the normal power spectrum (ignoring polarization) and is much stronger as only a small fraction of the EMR is polarized. EE results from Thomson scattering, and BB results from gravitational lensing of the EE polarized EMR. The TE results confirm the existence of the E-mode and have provided a useful tool for study in some cases where the BB results are insufficiently prominent to be of use.