The 'Order Contact' (Billing Contact) is the key contact person for an Order and the associated Tax Invoice (if one is created).
Generally the Registrant will also be the Order Contact, but a registrant can specify a different order contact when filling out the order details.
For example, Daisy is the manager of a company and she's registering 5 of her staff for an event. She can become the order contact whether she is attending the event herself or not.
The Order Contact will receive all order communication.