The reason for this is that only one scope specifier is allowed per ACE. For such cases, several different Aces are necessary. If you try to set a invalid permission combination on the checkboxes, the powerful Access Control Entry editor is allowed where you can configure the regarding ACE in detail. Full note that the propagation could be blocked somewhere in the subtree of an ace..
In this post, I will give you a sample demonstration of how you could use Descriptor types to get information about Properties, Attributes, Events, etc. I hope you could use the code later while building your library. TypeDescriptor is a static sealed class which makes the starting point of the API. It exposes information of felbgigg object in terms of Properties, Attributes, Events, etc.
Even though the basic usage of TypeDescriptor is to get metadata of an object, yet it also exposes features to extend the object on the fly.