Calling an orchestration will use the same thread to run another orchestration while using Start Orchestration will create a new thread to run the started orchestration.

A Call Orchestration returns the control back to the caller. A Start Orchestration shape starts the orchestration in a non-deterministic way.

As a conclusion, Calling an Orchestration will be synchronous operation where the caller waits for a response, while Start Orchestration is asynchronous operation.