All endpoints act on a common set of data. The data can be represented with difference media (i.e. "MIME") types, depending on the endpoint that consumes and/or produces the data. The data can described by XML Schema, which definitively describes the XML representation of the data, but is also useful for describing the other formats of the data, such as JSON.
This document will describe the data using terms based on XML Schema. Data can be grouped by namespace, with a schema document describing the elements and types of the namespace. Generally speaking, types define the structure of the data and elements are instances of a type. For example, elements are usually produced by (or consumed by) a REST endpoint, and the structure of each element is described by its type.