The act of an agent communicating (service provider, social media, etc) their arrival by registering/confirming for a previously reserved service (e.g. flight check-in) or at a place (e.g. hotel), possibly resulting in a result (boarding pass, etc). Related actions: - [CheckOutAction](https://schema.org/CheckOutAction): The antonym of CheckInAction. - [ArriveAction](https://schema.org/ArriveAction): Unlike ArriveAction, CheckInAction implies that the agent is informing/confirming the start of a previously reserved service. - [ConfirmAction](https://schema.org/ConfirmAction): Unlike ConfirmAction, CheckInAction implies that the agent is informing/confirming the *start* of a previously reserved service rather than its validity/existence.