The {class}Acts::Layer class is an extension of the {class}Acts::Surface class that allows the definition of sub surfaces (sensitive surfaces for modules, or extra material surfaces).

The layer can simply correspond to a 'virtual' surface in the detector description or represent a more complex object that may contain:

• a representing surface, which is accessible via a {func}Acts::Layer::surfaceRepresentation
• an array of contained surfaces, accessible via {func}Acts::Layer::surfaceArray method
• approach surfaces (i.e. boundary surface of the volume occupied by the layer)
• surface material description on any of the confined surfaces

The following illustration shows an $xy$ view of a cylinder layer with planar detection modules:

Modules can be sorted onto layer using all supported binning methods described through the {class}Acts::SurfaceArray class. The binning can be adjusted to fit as well as possible.

The unoccupied space in a volume that contains a layer array is filled with objects of type {class}Acts::NavigationLayer, which allows that in a fully static geometry setup, every single point in a volume can be associated with a layer. Layer objects are confined together in a special {type}Acts::LayerArray class and can be contained by a {class}Acts::TrackingVolume.