A transparent surfaces in general produce both reflected and refracted light.
It is assumed that for simple transmittance, the incoming light comes from directly beyond the transmitter. This is a reasonable assumption when the front and back surfaces of the transmitter are parallel and the thickness is not great, e.g., for a pane of glass. A transparent surface can be treated as a blend color. When blending, the transmitter's color is mixed with the incoming color from the objects seen through the transmitter. For other transparent media, the refractive index by Snell's law plays a role.
