DBCLIENT.DLL contains the low-level implementation of TClientDataSet. It manages all of the data and implements functionality like filters, sorting, etc. The relationship between TClientDataSet and DBCLIENT.DLL is very similar to the one between TTable and the BDE. The functionality is surfaced through two COM interfaces (DSBase & DSCursor) and requires OLE registration. If DBCLIENT.DLL is placed in any directory on the path or in the EXE directory on the client machine, it will automatically be registered by the VCL code in TClientDataSet. Otherwise it must be registered using REGSVR32.EXE or TREGSVR.EXE.