In practice, the physical tree is not represented as such in the data part of the program. Instead, a logical tree structure is defined, the JVOLUM data structure, which describes the arrangement of volumes in a compact and recurrent way. Each generic volume appears once, and once only, and carries the information relevant to the volume itself and to its contents, if any, by reference to the generic volumes corresponding to those contents. In the situation where division or multiple copies occur, there is no longer a one-to-one correspondence between a volume in the logical tree and a region in space. Information has to be added at tracking time to identify which division cell or which copy was considered at each depth level along the path through the physical tree. This information is stored by the subroutine GMEDIA/GTMEDI, for the current point of the current track, in the common /GCVOLU/. It includes the current level number NLEVEL and, for each level, starting from the first mother volume, the identification of the corresponding volume, e.g.: NAME, NUMBER, 'ONLY'/'MANY' flag, translation and rotation with respect to the master reference frame and so on. The number of shape parameters and pointers to the vector describing the actual values of those parameters, for each level, are stored in the data part and in the link part, of the data structure JGPAR, respectively.