Hi Martina,
In general, all structures in GeoDict are voxel structures. Additionally, structures created by GeoDict's structure generators (Geo-modules) also contain an analytic description based on objects (e.g. fibers or grains). These objects are sometimes called GAD-objects (short for "GeoDict Analytic Data"). One of the advantages of GAD-objects is that they can be edited later on: For example, a fiber can be rotated, shifted, its diameter can be changed or it can be deleted from the structure.