The Can be Started item refers to all assigned incomplete tasks that either have been started or that can be started given all predecessors have been completed.
The To Start Soon item refers to assigned tasks that are still in a “Waiting” status with an Estimated Start Date that is approaching (the number of days used is the same as for the “To Start Soon” task reminder that is defined in the account settings).
The Soon Due item refers to assigned incomplete tasks with an Estimated End Date that is approaching (the number of days used is the same as for the “Soon Due” task reminder that is defined in the account settings).
The Overdue item refers to all assigned incomplete tasks with an Estimated End Date that is in the past.
The Task Reminder settings are located under Administration => Account > Edit Account.