Editing XML Schema (W3C) (section)

== Schemas and schema documents ==
Technically, a '''schema''' is an abstract collection of metadata, consisting of a set of '''schema components''': chiefly element and attribute declarations and complex and simple type definitions. These components are usually created by processing a collection of '''schema documents''', which contain the source language definitions of these components. In popular usage, however, a schema document is often referred to as a schema.

Schema documents are organized by namespace: all the named schema components belong to a target namespace, and the target namespace is a property of the schema document as a whole. A schema document may ''include'' other schema documents for the same namespace, and may ''import'' schema documents for a different namespace.

When an instance document is validated against a schema (a process known as ''assessment''), the schema to be used for validation can either be supplied as a parameter to the validation engine, or it can be referenced directly from the instance document using two special attributes, <code>xsi:schemaLocation</code> and <code>xsi:noNamespaceSchemaLocation</code>. (The latter mechanism requires the client invoking validation to trust the document sufficiently to know that it is being validated against the correct schema. "xsi" is the conventional prefix for the namespace "http://www.w3.org/2001/XMLSchema-instance".)

XML Schema Documents usually have the filename extension ".xsd". A unique [[Internet media type|Internet Media Type]] is not yet registered for XSDs, so "application/xml" or "text/xml" should be used, as per RFC 3023.