function OpenLoadStream: TIdStream; override;

TStream - Stream used to access the contents of the message part.

OpenLoadStream is an overridden TIdStream function that implements the method used to open and initialize the internal stream needed for loading an in-memory attachment before transmission. 

OpenLoadStream is used to ensure that the internal stream for accessing data in the message part is positioned prior to reading values during message transmission. OpenLoadStream stores the initial position in the stream, and resets the current stream position to its beginning. 

During transmission, values in the internal stream are encoded based on the properties specified in the message or message part using methods in TIdMessageClient like SendHeader and SendBody. 

CloseLoadStream is used to ensure that the internal stream for accessing data in the message part is reset when all values have been read during transmission. 

The return value for the method is the stream indicated in the DataStream property.