There are two icons for the notifications. Each icon is associated with a different type of notification.
Action Required: This is for the notifications that require urgent attention. For example, when a user submits a change request.
Important: This occurs when the event creator/owner sends a message to the attendees.
Informational (this type of notification does not have an icon): This is for general event changes. For example, new agenda item, new hotel, address change, etc.