An explicit, or user-generated, transaction is a single set of BEGIN TRANSACTION/END TRANSACTION statements surrounding one or more additional SQL statements.
A BTEQ .LOGOFF command following a BEGIN TRANSACTION statement and one or more additional statements also forms an explicit transaction as well as terminating the current session.
A .LOGOFF command aborts the transaction and rolls back the processing results of all the statements that occurred between BEGIN TRANSACTION and .LOGOFF.
All other Teradata session mode transactions are implicit.