The {class}Acts::Volume class is a container of {type}Acts::BoundarySurface objects, where each {type}Acts::BoundarySurface is an extension of the {class}Acts::Surface class with additional information about the attached volumes. The normal vector of the surface defines an inside (opposite w.r.t. the normal vector) and an outside (along w.r.t. the normal vector) direction. Either a single volume or an array of volumes can be attached to a volume.

The simples volume class is just a collection of surfaces, where the {class}Acts::TrackingVolume describes a volume that can contain:

• an array of contained layers
• an array of contained volumes (as a container volume)
• an array of contained volumes (as floating objects)
• a volume based material description

The shape of the volume is defined by {class}Acts::VolumeBounds classes that create the corresponding bounding surfaces and register the attachment to the volume itself at creation.