You will see a check mark in this column if an appointment has an alarm reminder set with the appointment item.
You will see a check mark in this column if the appointment is linked (or associated) with a contact in your contact list.
You will see an icon of a page to indicate that you have some notes that are attached to the appointment item. A single click in this column will open the appointment item directly into the notes editor.
This help article applies to Time & Chaos 7