To me, project support material is associated with a specific project or projects, while reference is stuff that you may or may not want in the future. But the same thing might be one or the other, depending on the situation.
So let's say that at some point I found a list of tomatoes known to be suitable for dryfarming. If I just found it, said, "Yay!" and put it away in case I want it someday, I'd say it's reference.
On the other hand, if I planned, as a project, to plant a big bed of dryfarm tomatoes, and I went hunting on the web to make a list, and I used the list while hunting for tomato seeds or vendors of tomato plants, then I would regard that list as project support material. The diagram of how I plan to plant the tomatoes would also be project support material, as would any shopping lists, and the schedule for raising tomato seedlings, and so on.
As another example, a recipe is project support material if it's the thing I'm gong to cook for that party on Friday. It's reference if I just think it might come in handy someday. The notes that I took after making it, editing the recipe for a better version next time, are also reference.
If I imagine the difference in a visual way, I see reference as the file cabinet and bookshelves, while project support material is the stuff I pulled out of the file cabinet and put in a folder, along with the stuff that I just now wrote or sketched, because I'm going to use that stuff to guide an active project.