In the Comment Line table (97) there is a code field. Currently this field is without a related table.
Many of my customers have a change, where we have added a Comment Code table. This allows them to categorize their comment, and use the comment by filtering the different codes.

As it is not currently possible to add a table relation, to an existing field, then please add a new table/list page (Comment Code) and use it for the code table relation. The table should just have a code and description field.