Does anyone know the difference between these two icons in MediaBay, both of which are labelled as “Plug-in Presets”? I can’t find any documentation anywhere describing what they actually mean. The square icon signifies “Program” in Halion parlance, and is also to be see in Groove Agent, but they do appear to be largely interchangeable and I can’t see any unique characteristic that differentiates one from the other.
I’ll answer my own question 
The solid white symbol means “Multi” when referring to a Steinberg product (Groove Agent, Halion etc), but it’s just a regular preset for any non-Steinberg product.
The hollow square icon seems to be Steinberg-only, and means “Program”. It’s a wafer-thin distinction, frankly, and seems pointless to me.