Hard Drive Testing with Dell Diagnostics

We are testing some Hard Drives using Dell Diagnostics on a R510.

There are different tests.  SMART Test.  Short Self Test.  Confidence Test.  Buffered Test.

We have some drives that pass Smart, Buffered and Short Self Test.  However they fail Confidence Test.

What does this mean?  Do we need to be worried? Are we testing them properly?

Thank you