Two types of material description exist, one for a surface based material, one for a volume based material. They will be dealt with differently in the extrapolation.

The basic information for any material is:

the radiation length X0 the nuclear interaction length L0 the atomic weight A
the atomic charge Z the density of the material

This information is confined together in the {class}Acts::Material class.

In track reconstruction, only an effective material description is needed, i.e.
non-physical values in regards of the atomic number, the elementary charge or
even the density are allowed, as long as the effective relative radiation
length and $A/Z \times \rho$ ratio can be retrieved.  This enables the compactification
of the material description, as the element composition record does not have to
be kept.

Surface based material extends this material information by a representative thickness; the corresponding object is called {class}Acts::MaterialSlab. The thickness hereby can be arbitrarily chosen in order to regulate the material budget, it does not have to represent the actual thickness of a detector element. To attach it to a surface, a dedicated {class}Acts::ISurfaceMaterial class (or it's extensions) is used, which allows to also describe binned material.

Possible extensions are:

{class}Acts::HomogeneousSurfaceMaterial, homogeneous material description on a surface
{class}Acts::BinnedSurfaceMaterial, an arbitrarily binned material description with a corresponding {class}Acts::BinUtility object

In addition, a dedicated extension exists to allow configuration of the material mapping process, that is in further described below.

{class}Acts::ProtoSurfaceMaterialT, only binning description (without material) to be used in the material mapping process, which can be specified with a templated binning description.