Define the difference between core types and extended types

[evanp]

In section 4 "Model" we say:

The object types defined by the vocabulary are segmented into a set of eight core types and an extended set of Activity and Object types common to many social Web applications.

Either here, or in the vocabulary, we should explain what the difference is between "core types" and "extended types". Why do we need one or the other?

I think the difference is that "extended types" are the types that actually matter in the social networking problem domain, and "core types" are abstract super types for these extended types. These abstract supertypes let us define common properties, as well as ranges and domains that can cover many extended types.

[evanp]

The problem here is that some of the "core types", like Collection and Link are commonly used in real applications, whereas others like Activity or Object are rarely used, and only their subtypes are seen in the wild.