XML Schema Complete Reference, The

6.5. Constraining Elements

Perhaps the most important goal of the XML Schema Recommendation is to provide a language for constraining XML instances. A large part of this effort is constraining elements. An element type broadly specifies either simple content or complex content. An element type bound by simple content contains only a text value. This value may be constrained by built-in datatypes or other simple types derived from those datatypes. An element type whose content is complex might specify attribute types or nested element types (or both).

6.5.1. Simple Content

An element type that constrains an element to contain simple content places restrictions on the text value of the element. An element type that specifies simple content may ...