You can create subjects only visible in Community Portal. Or alternatively, you can use the same subjects that you use for Knowledge Base articles or tickets. If you use the same subjects, there will be a connection between the tickets, articles and threads.
First of all, to add a subject, please click on the "ADD NEW SUBJECT" link from the admin menu under "SUPPORT -> Subjects".
After clicking on this link, you will see the interface below:
If you want the new subject to be visible only in tickets, select only "Threads" in the "CONNECTION" field:
After you fill in the other fields, click on "Add Subject".
If you have a specific agent in your team, who is responsible or qualified in this subject, you can add this agent in the "ASSIGN TO" field.
This way, the selected agent will be notified when a thread is opened with this subject selected. If you add multiple agents, the system notify a randomly selected agent.