Like the other regulars ("Adobe Community Experts" who are not Adobe employees) who have also posted in your thread, I had never heard of "InDesign Pro." However, since you mentioned volume licensing, I was able to find a reference to this on CDW. That led me to look for something that was just an Adobe "Pro Edition," which led me here. It looks like the "Pro" designation might mean "easy management interface for your IT department to deploy stuff" as well as "unlimited access to Adobe Stock."
These are just guesses, though. I feel like we might need an Adobe employee in here to help us out with this one, as it looks like the kind of thing where you might need to be an enterprise customer to even be allowed to ask what it is! I've filled out the form, though, so maybe I'll be able to get someone at volume licensing to tell me what the Pro designation means, or perhaps they can tell someone else at Adobe so that we can get a brief summary from Official Sources that we can share with you.