The 'Specular' and 'Sheen Layer' settings are the same. It is recommended that you use 'Sheen Layer' for tight specular reflections and 'Specular' for broader specular reflections.
The color the specular reflection will be modulated with.
Sheen Layer (yellow) and Specular Color (red)
The specular weight.
Controls the glossiness of the specular reflections. The lower the value, the sharper the reflection. In the limit, a value of 0 will give you a perfectly sharp mirror reflection, whilst 1.0 will create reflections that are close to a diffuse reflection.
The amount of reflection will be dependent on the viewing angle of the surface following the Fresnel equations (which depends on the IOR value).
The visual effect is that the reflection increases as the viewer's angle of incidence, with respect to the surface normal, approaches 0.
The white powdery look, apparent on the grapes in the images below, is achieved using a high roughness value. Notice the effect that increasing the IOR has on the specular roughness.
There is currently an issue with Specular Fresnel (IOR) which produces a darkening effect around the edges of the Skin shader. This issue is unrelated to bump mapping. However, the effect is more apparent when using bump mapping because it increases the areas where there are grazing angles. This issue is due to be fixed in a future release. A workaround is to disable 'Fresnel Affects SSS' found within the 'Advanced' options of the Skin shader.
Bump mapping increases darkening effect caused by IOR