Represents a top of a geometrical sub-hierarchy not placed in space None of its children can coincide with its boundary defined by an associated solid Two different placements of the same logical volume represent two different geometrical hierarchies in space Allows to create a group of volumes bound together without a boundary All the volumes exits inside the same virtual reference system of the assmebly volume they belong to When assembly volume is placed all its children follow the global transformation applied to their assembly volume After the assembly volume is placed its children exist as standalone placements in space independent of each other Represents a single unique copy a of an associated logical volume in geometrical hierarchy Definitions of a geometrical hierarchy of a set of volumes Geometry setup representing the particular geometry hierarchy by refferring to a given volume which becomes the top level volume A reference to the previously defined volume in the structure block chosen by this setup World volumme can't be an assembly volume