There are two types of identifiers for the ATLAS calorimeters:<br>
  offline identifiers (type= Identifier), and online identifiers (type= HWIdentifier). <br>

And there are two 'families' of identifiers:
detector specific identifiers (for EM, HEC, FCAL and TILES)
calorimeter-wide identifiers (for Trigger Towers, for Dead Material and for EM/HEC/FCAL/Tile wrapped together in CaloCell)

All identifiers are decoded by specific classes called "Helpers", all deriving from base class AtlasDetectorID.
The field content of the identifiers is described in xlm files called dictionaries which can be found in the data folder of the DetectorDescription/IdDictParser package.

  The offline identifiers are decoded by the following helper classes:  LArEM_ID ,  LArHEC_ID ,  LArFCAL_ID ,  TileID
  and CaloLVL1_ID , CaloDM_ID , CaloCell_ID of the present package. <br>
  The corresponding dictionaries are IdDictLArCalorimeter_xxx.xml (LArEM_ID, LArHEC_ID, LArFCAL_ID), IdDictTileCalorimeter_yyy.xml (TileID), 
  and IdDictCalorimeter_zzz.xml (CaloLVL1_ID, CaloDM_ID); there is no dictionary for CaloCell_ID. <br>

  The online identifiers are decoded by the following helper classes (release 13 onwards): <br>
  TTOnlineID of the present package <br>
  LArOnlineID , LArReadoutModuleID of package LArCalorimeter/LArIdentifier <br>
  and TileHWID of package TileCalorimeter/TileIdentifier. <br>
  The corresponding dictionaries are IdDictLArCalorimeter_xxx.xml (LArOnline_ID,), IdDictTileCalorimeter_yyy.xml (TileHWID) 
  and IdDictCalorimeter_zzz.xml (TTOnline_ID); there is no dictionary for LArReadoutModuleID. <br>
  There are also identifiers/helpers for calibration lines and High Voltages, see package LArCalorimeter/LArIdentifier. <br>