Nesting of Property Types, for example, would allow an array of sign panels on a post to have Pay Item information attached to each sign panel.

So, if I have an Item type with an array Property Definition set to a Property Type, I can attach that Item Type to a sign post. Then, for each panel on the post, I can create an instance of an array for each panel. This works fine currently. What I cannot do is attach Pay Item information to each panel in the array, this would require Property Type nesting.