Some hashtags are defined by the system (and can't be changed as far as I know, though some can be 'tweaked' a bit.) When you check the "Send Invite To Group" box for an Event (default is checked), the message will be sent with the cal-invite hashtag. If you set up reminders for the Event, they'll be posted with the cal-reminder hashtag. If you choose the "Send Notice To Group When Event Happens" box, it will be sent with the cal-notice hashtag.
Thank you, Duane. Sometimes it's hard to understand how things work in IO. I need to test this feature a bit more. I'll put your explanation in my moderators' group Wiki.
