When you decide to use sub activities, you might want to prohibit registering time on the main activity. And when you are finished with a project, you might want to prevent accidental time registration on its activities. In those cases, you can disable the activities; this way you cannot make the mistake of registering time on them. Disabling and enabling activities can be done from the "Manage activities" window. This window can be opened by selecting "Activities \ Manage activities..." from the menu.