A Reflection Notes field is available for both the high-level question on assessment area and for each of the assessment topics. By default this notes field can be edited by all users, who have access to the assessment, via the permission set ARPEDIO assessment user.
You can restrict this access by cloning this permission set, and change the read / edit settings on the comments fields under Object settings for the new permission set.
The notes field for the high-level question in assessment area is in object ARPEDIO Assessment elements.
The notes field for each of the topics is in object ARPEDIO Assessment questions.