Absorption Layer Transmittance refers to the absorption of light at the transmitting surface by a given "layer", which allows further control over how specular materials appear without changing the medium's absorption properties (which is what usually creates the perceived colour).

This can be useful for producing a stained glass window effect for example.

An example material with a grating texture used as an absorption layer.

Materials which have an absorption layer attribute:
Specular
Glossy Transparent