T stands for transmission and takes into account the light absorbed (and scattered?) by the many elements of a zoom lens, so it depends on the lens. For the Angenieux 12-120, geometrical stop f2.5, it's T2.2 and so on. The loss of light also occurs in still camera lenses, of course, but the small differences can be corrected in printing, whereas in film, shot-to-shot consistency in density is much more important. Failing to allow for the T-stop would introduce an unnecessary variable to be corrected in printing or telecine.