While viewing the list of activities in Activity Center, the hierarchical view mode is enabled by default. The mode allows the hierarchy of activities you have added to a project to be visible to you. This is very much like the way Windows Explorer displays folders on a disk. You can expand an activity to display its sub-activities by clicking  ("Expand" button). Clicking  ("Collapse" button) collapses an activity or project similar to collapsing a folder in Windows Explorer), so that sub-activities/activities are no longer visible.

The alternative to the hierarchical mode is the flat view mode. Turning on the flat view mode disables the hierarchical view mode and vice versa.

Hierarchical View

To enable hierarchical view  (disable the flat view mode):

1. Open Activity Center.
2. Click the hierarchical view mode button ()
3. The button changes to active (depressed) state: 

Flat View

To enable flat view (disable the hierarchical view mode):

1. Open Activity Center.
2. Click the flat view mode side of the view mode switch ()
3. The button changes to active (depressed) state: .