Redmine

Every custom field in database has it's own ID in custom_fields table, and it's accessible thru API.

However, when type is LIST, list item values should be stored in separate table, because of relational database integrity.

If Redmine is to be integrated with some other software via API, and it needs to use custom fields of type LIST, integration can be
performed only by value not by value's ID, which means, if someone changes value, 3rd party application's database integrity is lost, if it were ID, then integrity won't be lost, as it's in every other table of Redmine, or any other relational database.