Currently there is only an option to send an email to "registered" contacts.
When sending "thank you" or "follow-up" emails for events, we need to be able to send to only "attended" event contacts - along those same lines, would be handy to send "not attended" contacts a separate email on what they missed, etc.
Basically, you have 4 event group options: "registered" "cancelled" "attended" and "not attended" and it would be logical to be able to email each group separately if desired.