Use Open Access when you want every registered attendee to be able to watch a specific talk, even if their ticket would normally exclude that talk.
When Open Access? is enabled for a talk, all registered attendees can access both the live broadcast and the replay, regardless of ticket restrictions. This is useful for keynote, sponsor, orientation, or promotional sessions that should be available to everyone who signs up for the event.
To turn it on, go to Content > Talks, open the action menu for the talk, choose Advanced Settings, then enable Open Access? in the Attendee Limits & Access section.
Attendees still need to register for the event before they can watch an Open Access talk. If you want a talk to be viewable without registration or a ticket purchase, use Public Access? instead. Public Access also disables live chat, Q&A, and polls for that talk.

