Use this window to register the different classes of activities and its values which can be used when you create a new activity. An activity class is only used to categorize activities and affects nothing else in the system.
Example: The project has 50 activities that you wish to categorize according to the department that would be handling them. You can define an activity class called Departments, and then enter the names of the departments (Finance, Procurement, Sales etc.) as the values.