The Layer Selection Section
Almost all physics nodes that you add in the Layered Shell interface have a section named Layer Selection. Selections there interact with the standard selections of geometrical objects (boundary, edges, or points) in order to provide a complete specification of where a material property or boundary condition is to be applied.
The physics features can be of two, fundamentally different, types: Those who act on the layers themselves, and those who act on the interfaces between layers. The settings for these two types of features are slightly different.
See Physics Interface Selection for the more details.
For a general description of layer selections, see Layer Selections in the documentation for the Composite Materials Module.