From the beginning, we've separated the idea of documents and nested types. The primary difference between these types is that:

• Document types define the shape of built content files
• Nested types define shapes of content used within built content files

We may be able to simplify the API by considering every type definition as just that — a type definition. We already have to define documentTypes when calling makeSource. Could this be the way in which documents are defined, while every other type is just a type.

#226 contains a way we could adjust terminology if making this change to help with the language we're using.