Region codes play a cardinal role in the geometry software. A user of the software must supply a coordinate to uniquely specify a point within the detector, but if this all that is passed then first the geometry would have to determine where the point is, which can be a very expensive operation. So every detector element is given a unique REGION CODE to be recorded in the central data structure along with each vertex coordinate. The geometry interface routines use the region code to determine which routine to call. The only exception to this (for obvious reasons!) is the REGION request. Region codes consist of a 3 digit REGION TYPE (which describes the type of region e.g. PMT) and a 4 digit REGION SERIAL NUMBER (e.g. PMT number). For more detail see:-
Companion/ID Codes/Regions
For each region (region type = n) the titles banks GEDP n and GEDS n holds the data that describe it.