If you choose to send notifications of changes to your calendar, there will be a few tags that are included in the subject headings.
If you have an event in draft status when you modify it, and you choose to publish and send the event, the message will use the New Event tag in the subject line. Currently, there is no way to turn off this automatic subject change. However, if you want to avoid this tag in the future, you can choose to copy your event, publish, and send the notification with the correct edits. Then, you would delete the original event.
The Modified Event tag is added to the subject line of any message that sends from an event that is already published and has been changed. This is a hard-coded functionality - it does not matter if you have sent a message before. If the event has been published and changed, a message will send with the Modified Event tag.