3rd Party Tickets are a subset of either Invites or Tickets. 3rd Party Tickets are unique in that the initial user (either the Invite recipient or the Ticket purchaser) is not necessarily the individual attending the event. Rather, they are the coordinator of the guests who will use the seats in the 3rd Party Ticket.
3rd Party Tickets are used for sponsors or event partners who may have multiple seats at your event and who want to manage those registrations themselves. 3rd Party Tickets can be created as Invites and added to emails, or created as Register Page Tickets and toggled ON to appear for purchase on your Register Page.