Using XACT_ABORT ON | OFF in SQL

CREATE TABLE t1 (a int PRIMARY KEY)
CREATE TABLE t2 (a int REFERENCES t1(a))
GO
INSERT INTO t1 VALUES (1)
INSERT INTO t1 VALUES (3)
INSERT INTO t1 VALUES (4)
INSERT INTO t1 VALUES (6)
GO

SET XACT_ABORT OFF
GO
BEGIN TRAN
INSERT INTO t2 VALUES (1)
INSERT INTO t2 VALUES (2) /* Foreign key error */
INSERT INTO t2 VALUES (3)
COMMIT TRAN
GO

SET XACT_ABORT ON
GO

BEGIN TRAN
INSERT INTO t2 VALUES (4)
INSERT INTO t2 VALUES (5) /* Foreign key error */
INSERT INTO t2 VALUES (6)
COMMIT TRAN
GO

/* Select shows only keys 1 and 3 added.
   Key 2 insert failed and was rolled back, but
   XACT_ABORT was OFF and rest of transaction
   succeeded.
   Key 5 insert error with XACT_ABORT ON caused
   all of the second transaction to roll back. */

SELECT *
FROM t2
GO

DROP TABLE t2
DROP TABLE t1
GO