Forgive me if this is vague...I thought I'd throw out my experience using a MSG instruction to write data from one PLC to another for unsolicted messages.
But, I had an application to where I needed to write data accross 2 PLC's.(Data Table Write)
From what I remember...
The Source tag is the first array element of the data being sent...(i.e. my_data[0])---Local
Then the number of elements is the amount of bits you are sending (i.e. 32)
And the Destination is the other computer (i.e. topic_name.my_data[0])---Remote