Several additional aspects of schemas don't fit too well into the preceding discussions.
Any element or type can be made "abstract" by adding an
to the "element" or "simpleType" or "complexType" element where the element or type is specified. Thus you cannot directly use the type or element in an instance of XML; rather, you can use it only in the specification of other types or elements. The "abstract" type/element has "false" as its default value.
You can insert "annotations" into a schema for human or automated consumption. These annotations use the xs:annotation element and are fairly obvious when they occur.