I didn't go searching for it very hard, I just took the link to the 2015 download, and changed the name to 2016 and there it was. The downloader was put on FTP July 11th, but the actual image of 2016 looks like it was put up there on June 29th!
Anyway yeah the Ack appears to be a special channel that you give the ID of a stream and then wait on the confirmation from the writeing of the Ack. As a result the data type can't change, and it is always the data type of the ID which is probably an I32.
There are a few good examples, but none that I saw mention the Ack functionality, even the Messenger one just shows how it works with a mouse tracking example.