From Xojo Documentation
Occurs when your network provider queues your data in chunks and is about to send the next chunk.
The parameters indicate the amount of progress that has been made during the send. Returns a Boolean.
Returning True from this event causes the send to be cancelled. This does not close the socket's connection; it only clears the buffer. After all of the data has been transferred you will get a final SendProgress event followed by a SendComplete event.
bytesSent is the number of bytes that were sent in the chunk, not the total number of bytes sent.