SPECULAR

Reflection Tint

Colour multiplier for external reflection. It should be used sparingly, as a non-white value breaks physicality.

Transmission Tint

Colour multiplier for rays going inside the medium (covers external transmission and internal reflection). It should be used sparingly, as a non-white value breaks physicality.
The prefered way to define the colour of a dielectric is through the Interior attributes right below.

Fresnel Mode

Switches the Fresnel between Artistic and Scientific parameters.

...

When reflectivity90 is 1, then both modes achieve identical results, as one can be mapped to the other.

Reflectivity

Colour reflected by the surface in the normal direction. Corresponds to specColor.

IOR

Index of refraction (often denoted by "eta"), defining the amount reflected by the surface in the normal direction, and how the rays are bent by refraction. A separate value can be specified for each colour channel, if desired.

Reflectivity 90

Colour reflected by the surface at the grazing angle. Should be kept at 1,1,1 for most materials. Corresponds to specColor90.

ReflectivityProfile

 The inverse of the exponent to use for Schlick

...