(Transmission factor is an old and deprecated term for transmittance.)
(Terms of radiometry/photometry)
Transmittance is the ratio of the total radiant or luminous flux transmitted by a transparent object to the incident flux, usually given for normal incidence.
The ratio of the directly transmitted light after passing through one unit of a participating medium (atmosphere, dust, fog) to the amount of light that would have passed the same distance through a vacuum. It is the amount of light that remains after the absorption coefficient and the scattering coefficient (together the extinction coefficient) are accounted for.
In the Radiance software, those units are used in the specification of several material types: