What is mean AHR2?

Dear friends, I am new at ardupilot.
I try to data log analyze according to arducopter website. But I was confused some headline that like AHR2 and NKF.
How can I learn which result related which block of sensors?
Would you help me please?
I shared with you it that I was confused data review.

Best Regards.

have you found the anwser,guys?i have same problem now