A Continuation is a way of making long-running callouts from a Visualforce page without blocking the user interface or exceeding the governor limits. A Continuation can chain multiple callouts and return the responses to the same Visualforce page. A Continuation is suitable for this scenario because the billing and tax information callouts are dependent on each other and might take up to two minutes each. An HTTP REST callout is a synchronous callout that blocks the user interface and has a limit of 120 seconds. Using an HTTP REST callout for either or both of the callouts would not meet the requirements of displaying the information at the same time and avoiding timeout errors. Reference: [Continuation Class], [Callout Limits and Limitations]