The AuthoritExtensibility namespace contains types that you use to communicate between your plugins and Author.
The AuthoritExtensibility.dll assembly contains the AuthoritExtensibility namespace. You can find this assembly in the Author-it installation folder. By default, C:\Program Files\Author-it\ is the installation folder path on a 32-bit operating system and C:\Program Files (x86)\Author-it\ is the installation folder path on a 64-bit operating system.