The user may define volumes which have no relation with the real objects. It is sometimes convenient to make use of such volumes, to artificially delimit regions with simple shapes. Doing this, it may happen that volumes partially overlap each other. (A volume positioned inside a mother is obviously not regarded as overlapping the mother).
It is possible in GEANT to handle partially overlapping volumes, but there are some implications that the user should be aware of. A flag 'ONLY'/'MANY' is assigned to each copy of a given volume at the moment this is positioned. The 'MANY' option indicates that a point found in this volume could also be in other volumes which are not its direct descendants. If one of the overlapping volumes where the point is found is declared as 'ONLY', the geometry subroutines will give priority to this volume, and the search stops. If a point is inside several 'MANY' volumes and outside all 'ONLY' ones, priority will be given to the volume found at the deepest level, or to the first found, if there are many valid 'MANY' candidates at the same level. In order to avoid ambiguities, two overlapping 'MANY' volumes should in general have the same tracking medium.