xsd:ID

The type xsd:ID is used for an attribute that uniquely identifies an element in an XML document. An xsd:ID value must be an NCName. This means that it must start with a letter or underscore, and can only contain letters, digits, underscores, hyphens, and periods.

xsd:ID carries several additional constraints:

• Their values must be unique within an XML instance, regardless of the attribute's name or its element name.
• A complex type cannot include more than one attribute of type xsd:ID, or any type derived from xsd:ID.
• xsd:ID attributes cannot have default or fixed values specified.