In Activity Center you select projects using checkboxes next to their names. Checking at least one project selection checkbox will immediately make the batch operation panel visible.

To allow for quick selection of activities or sub-activities under a project, you can use special menu available for projects with activities.

The menu is accessible via a downward arrow beneath the checkbox next to a project (with activities), which appears when the pointer is placed on the checkbox ().

Clicking the arrow  opens a context-sensitive menu. When applied to projects it allows checking/unchecking all activities (shortcut: Click + Ctrl). When applied to activities, checking/unchecking all sub-activities is available.

Since the menu is context-sensitive, if a project has already all activities selected, the menu will contain only one command: Uncheck All Activities (Click + Ctrl).

Selecting a project along with an activity is not possible. When you select a project by checking its checkbox, all the checkboxes of its activities become invisible. Check out the checkbox status chart below.

Selection Checkbox Status Chart