What is the ATC service advertised in Bonjour?
The _atc._tcp. service is/was the Shared iTunes Library service.

I'm out and about at the moment, but when I get a chance I will update this answer with some sort of link/source to back this up.

[EDIT]

I haven't yet had a chance to have a thorough look for a source, although I did do a quick Google search and found a reference to the _atc._tcp. service at this MacRumors discussion. In it you'll see the 10th comment (by priitv8) states:


  For comparison, I've included a screenshot from my Shared iTunes Library service (_atc._tcp.).


Also, it occurred to me that you may want to download Bonjour Browser and see if that sheds any more light on this for you. It's an old app but works well (although I haven't used it in macOS Sierra - not sure what you're using).

As an aside, the list of Bonjour service types used in Mac OS X you linked to is not an exhaustive list. It was only a list of common Bonjour service types used in Mac OS X and hasn't been updated since July 2004.