Add Comments to Dimensions and Members
You can add comments to Essbase dimensions and members. Comments can contain 255 characters maximum.
To add a member comment using the Essbase web interface, open the outline for editing, select a member, and edit its general properties. Enter text into the Description field as appropriate.
If you are using an application workbook or Cube Designer to add or edit a member comment, update the COMMENT property on the relevant dimension worksheet.

If you use REST API to edit the outline, you would update the value of the comment property of mbrInfo to be the comment string, when performing a member add or update action, as documented in the endpoint for Run Batch Outline Edit.