Well, I would assume that the package class means … it’s a package. The same way a UObject is a UObject. I really don’t understand what the problem is? Never used the class, but by the name and members like AccessThumbnailMap() e IsFullyLoaded(), I would assume it most likely refers to an unreal file package.
Now, in relation on where and how is being used, well, based on my assumption, I would said thats rather clear, right? You create, load, save, delete … packages containig textures, sounds, and so on. Therefore UPackage should also be a serializable class.