As shown in the attached screenshot, in our recent Learn Moodle 3.4 Basics MOOC, the URL field is used when listing database activity entries on the tag page. This is confusing, as the link goes to the database activity entry and not to the displayed URL.

It seems that it is currently not possible to specify which field should be used.

It would be helpful if a teacher could specify the field, say by having a checkbox (with explanatory help popup) on the edit field page, as suggested by Andrew Hancox in MDL-57455.