A top-level geometry is the starting point of a new geometry tree. It usually represents the part of a device where it is mounted to any kind of support structure, e.g. the base of a moving light fixture or the yoke of a PAR can.
A child geometry is a subordinate geometry inside a geometry tree. It always depends on its parent object. Each child geometry has exactly one parent geometry and it can have multiple child geometries itself.
Child geometries represent parts of a device which are physically and/or logically connected to their parent object.
The position and rotation of a child geometry is always relative to its parent geometry.
Furthermore, control and visualization applications can make use of the geometry structure to inherit controls from a parent object to its children or vice versa.
To transform a top-level geometry into a child geometry, drag and drop it on top of the desired parent object. This will make the geometry a child of that parent object.
To transform a child geometry into a top-level geometry, drag and drop it on the + Add Top Level Geometry or + Add Child Geometry labels.