The CMB “optical depth to reionization” t measures how many CMB photons were rescattered by free electrons produced during reionization, and in ΛCDM it is expected to be a single, well-determined global parameter tightly linked to a relatively smooth ionization history (Kaplinghat et al. 2003; Planck Collaboration VI 2020). In practice, low-l polarization data, different analysis pipelines, and combinations with external datasets such as BAO and large-scale structure produce somewhat discrepant or scattered t estimates, hinting that either reionization is more complex and patchy than the simple ΛCDM templates assume, or that subtle systematics and degeneracies with other parameters (like A_s and O_m) are at play, making it hard for the standard model to consistently reconcile all t determinations within one neat, minimal history (Haiman & Holder 2003; White 2021).