The primary subject on a case is considered to be the focal point of the investigation, and Trackops will display the subject's name on several notable places throughout your system. In addition, the search function will allow you to quickly find cases by primary subject.
In order to change (or set) the primary subject on a case, it important to know that only subjects marked as the default subject type are eligible to be marked as primary. The ability to mark a subject as primary will not be present on the subject form if it is not of the default type.
Note: If you aren't sure of the default type, or need to make changes, you can configure the default subject type setting on the related case type.
Once you have confirmed that the subject is eligible to be marked as primary, navigate to the subject's profile and click the Edit button in the upper right. From here, check the Make Primary box, located at the top of the form.
Finally, save the subject form, and you are done. The new subject will now appear on the case and the case list as the "Primary Subject".