Were you referring to BG_ESTIMATED_FIX_TIME?? I didn't see any system field as FIX_Days in the Bug Table.The dependant fields of estimated fix or actual fix are auto calculated by ALM. the actual system code behind this logic for these fields or anyother autocalculated fields will not be displayed\done at workflow level as well didn't see any specific info in DB API.Probably you may check with HP or some one else can throw more light.
FYI from DB Reference
BG_ESTIMATED_FIX_TIME
The number of days the developer estimates will be required to fix the defect.
BG_ACTUAL_FIX_TIME The number of days worked to fix the defect.