Let’s explore together quickly what they are all about; we’ll start with the sheen parameter.
The sheen parameter is useful when you have to simulate materials such as fabric and velvet. Before this, we used to simulate the change in color on the side of these materials hit by the light, with the use of a fall-off map on the diffuse using 2 colors: one for the front and another one, brighter than the first one, for the side. With the sheen option, we can make everything directly from the material.
So to simulate a velvet material for example we set the diffuse color, then we go inside the sheen option and we set the sheen color brighter than the diffuse (I normally set the sheen color 30% brighter than the diffuse as a starting point).
The sheen glossiness parameter is the one that makes your fabric more or less shiny (silky):
if the value is high, for example 0.9, the effect will be very subtle like it usually happens when you are trying to simulate a fabric like jersey. To replicate the silk, the sheen glossiness has to go very low like 0,4 or less.
Here you can find examples of different sheen glossiness values.
Decreasing the amount results in more visible sheen color on the side of the material and on its illuminated part.
It is also possible to map the sheen glossiness value with a black and white texture. In this case, the texture will act as a mask where the black color will not reveal the sheen color and the white color will. So what we can do is to add both the bump map we are using to simulate the fabric texture and the sheen glossiness map so this will make the appearance of the fibers more visible.