It's mainly based on whether the asset should have jobs or not since parks don't have jobs and unique buildings do. For the most part, the larger ones are unique and the smaller ones are parks.

I like these a lot and the idea of putting them into the tourism / leisure tab is great. However, there does not seem to be any pattern as to why some are unique buildings and others are parks. I would have thought the larger stadiums should be unique and the small ones as parks.

Let me know if any problems with the stats. I'm still tweaking the formulas that generate all the stats.