Since all lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), T-stops are sometimes used instead of f-stops for exposure purposes, especially for motion picture camera lenses. The practice became popular in cinematographic usage before the advent of zoom lenses, where fixed focal length lenses were calibrated to T-stops: This allowed the turret-mounted lenses to be changed without affecting the overall scene brightness.
Lenses were bench-tested individually for actual light transmission and assigned T stops accordingly (The T in T-stop stands for transmission), but modern cinematographic lenses now usually tend to be factory-calibrated in T-stops.
T-stops measure the amount of light transmitted through the lens in practice, and are equivalent in light transmission to the f-stop of an ideal lens with 100% transmission. Since all lenses absorb some quantity of light, the T-number of any given aperture on a lens will always be greater than the f-number.
In recent years, advances in lens technology and film exposure latitude have reduced the importance of t-stop values. So, F-stops are for focal ratio, T-stops are for transmission.