Creates a new EDI functional group header segment on the current document.
When called, a new functional group start (GS or UNG) segment is created and set as the current segment. You should then set the values on said segment before adding a new transaction set within the document.
This method returns a result code; 0 indicates success, while a non-zero error code indicates that this method encountered an error during its execution. If an error occurs, the GetLastError() method can be called to retrieve the associated error message. (Note: This method's result code can also be obtained by calling the GetLastErrorCode() method after it returns.)