Difference between "object rm" and "object unlink" ?

[zhu_qiang_ws@xxxxxxxxxxx (zhu qiang)]

Hi all,
   From radosgw-admin commond :
  # radosgw-admin object rm --object=my_test_file.txt --bucket=test_buck
  # radosgw-admin object unlink --object=my_test_file.txt --bucket=test_buck
  The both above did  "delete"   the object  from bucket, both got the right result.
  What is  the difference between these two commands ?  Could some one tell me the detailes ?
Thanks very much.