Er is the relative permittivity. In the model card, this can end
up being a "fudge factor" that is only loosely related to the
material system.
You often see cases where the "optical" Tox and the
"electrical" Tox have diverged. Things like the density
of the grown oxide (if it is in fact even a simple oxide
and not some combination of nitride, hydrogen-loaded
oxide, high-K or whatever) affect the electrical qualities
of the oxide.