The Undecided List shows the scouts and adults who have not yet indicated whether they plan to attend the event.
You'll find the button that produces this report on the detail page when you select an event from Maintain Events.
Click the Print Undecided List button to produce the report. This button may also be found at the bottom of the Troop Calendar detail page.
The report is sorted by patrol, showing each scout and each adult who has not yet indicated whether they plan to attend.