When you access the Teradata server through FICON channel from IBM mainframe environment then we have to use TDPID to logon.
/* If you fail to mention TDPID then the mainframe will assign the default TDPID, it can be TDPA or TDPC*/
If you access the Teradata server through Local area network (LAN) from UNIX or Windows OS then we have to use the hostname to logon. Here we can also use the ipaddress of the server instead hostname of TD server.
The USERID will remain the same in both the case.