The GPDs used by the system are found through a chain of GPD maps formed from the linkitems of SYSTEM:GPDmaplist in the user and the system root objects. The search for a GPD tag thus begins in the map SYSTEM:GPDmaplist:1 of the user root object, and thereafter the maps :2, :3, etc. are searched in order. If the tag is not found there, the search is repeated for the system root object. Each GPD map looks something like figure 7.31.
As you see, the GPD map holds a table that defines tags for GPD objects and a brief description of the GPD. It is this tag which must be the same as the topmost concept in the STRUCTURE section. The table is contained under the link group GPDmap, where each field defines the association between a tag (the field tag) and a GPD (the object linked to at position 1).