Menu > Admin/Manager > System Setup > Global Lists > To Do Status
NOTE: This list is blank by default, the above is an example of how one might set this up.
Status - This labels the To Do Status so users know what status its in.
Mark As Completed - Multiple statuses can have this checked. For example, you might have Completed and Cancelled as Status labels, yet both can mark the To Do as complete.
Default - Only one Status can be the Default used for all new To Dos.