Geometries are the physical description of parts of the device and describe their physical and logical dependencies.
For example, a conventional par only uses the body and the beam as geometries.
The base is the top level geometry.
The yoke is the child of the base, the head is the child of the yoke and the beam is child of the head.
When using a fixture template or customizing an existing GDTF file, the relevant geometries are already available and only need to be adapted.