Activity classes are used to categorize activities. The registered classes and its values are available when you create a new activity. The activity class and value is for information only. Create an ativity class in Project Basic Data/Activity Classes page and create a new record in the Activity Class Values section to specify the values that are valid for each activity class.
Validity of the activity class defines whether the activity class needs to be used globally (for all projects in all companies) or per project. If the validity is set to project you also have to specify for which project it should be used.
An activity class can be blocked for use by selecting the Blocked for Used slider switch.