What does the Material element represent?

The Material element in gbXML allows you to define a building material such as a wall, roof, floor, window, ceiling, or any other construction type.The child elements allow you to define all properties of the construction type including its R-value, reflectance, density, thickness, dimensions, and much more. Here is a list of all of the child elements:

Description
Absorptance
Roughness
Albedo
Reflectance
Transmittance
Emittance
ImageTexture
R-value
Thickness
Conductivity
Density
SpecificHeat
Permeance
Porosity
RecycledContent
Fire
Cost
IndoorAirQuality
CADMaterialId
Reference

In fact, Autodesk Revit MEP stores many of its built-in constructions type descriptions in gbXML-formatted text files that accompany the software.