No, this never was documented, AFAIK.
Even when the Tools Manual existed and showed you how to use this feature, there was no documentation of the details of how it all worked internally.
With its predecessor - the old TMT and TE - it was also possible to pass data through the tools to another COM port on the PC.
It is all set by the configuration options that you set for the connection in Design Studio.
I have made some posts in the past, but SiWi seem to have lost them! 
eg, If you search for “cake”, the results will include a post titled “Have your cake, and eat it!” but, if you click the link, it will say, “The requested topic does not exist.”
And, if you click the linked topic within that post, it just goes to the forum index.
You could try speaking to your Distributor to see if they can get you an old copy of the Tools Manual…