How did sea ice fraction measured?

Hi, I am referring to sea ice area fraction variable from Arctic Regional Reanalysis on single levels from 1998 to 2019 dataset. I was trying to understand how this sea ice area fraction was measured but I could not find a good resources to refer to. I could not find any detailed information from both User guide and full system documentation and am wondering if anyone has a recommendation for this. Thank you! 

Did you look at Would this help?