Positioning of entities in superordinate coord. system

I was wondering if there is any clear definition on the placement of entities in a superordinate coordinate system (e.g. world/road/lane coord. system).

From using esmini and other tools I just assumed that they are (probably?) placed at the origin of the entity-specific coord. system (e.g. vehicle: ground-projected center rear axle; misc object/pedestrian: center bounding box but not ground-projected?; fog: center of bounding box even if there is no coordinate system for fog?).

It is clearly defined what the origin of each coordinate system is, but I did not find any clear definition on the relation between an entity's position values and its actual meaning (what does the position refer to). Did I miss something?