Categories are usually pre-defined by your system administrator and each category may include one or more subcategories. Subcategories are listed under the main category in a drop-down list.
How Categories Affect the General Page
Categories are also associated with groups of custom fields that appear or disappear based on the categories that you select. If defined, these custom fields appear on the General page, in the Details section.
For example, you may have a project that you assigned to a category named Medical Malpractice. Once you assign a project to this category, the Details section on the General page may display custom fields that allow you to enter details about the incident, such as the name of the doctor, the date the incident occurred, and the name and location of the hospital.
Based on your access rights, you can view, add, change, or delete categories.
See Setting Category Details for more information.