This is a Label's list. The Labels field is a system defined field in modules like Requirement and Test Case. The field is most often used for a major classification of a product or service. Some organizations use “Module”, "Component" and "Product" in place of Label.
Expand Projects on the side bar and select Labels.
Click on the "+" symbol at top right corner.
It displays fields inline to add a new label.
Enter Alias (i.e. the name that will be displayed) and Name of Label and click Save.
The Labels list appears on the Create screen for Requirement and Test Case.
Go to the respective modules and open detail page. Check the Label(s) drop-down with values you entered above.