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#### 6.2.4 Using type identifiers in schema locating files

Type identifiers allow a level of indirection in locating the schema for a document. Instead of associating the document directly with a schema URI, the document is associated with a type identifier, which is in turn associated with a schema URI. nXML mode does not constrain the format of type identifiers. They can be simply strings without any formal structure or they can be public identifiers or URIs. Note that these type identifiers have nothing to do with the DOCTYPE declaration. When comparing type identifiers, whitespace is normalized in the same way as with the ‘`xsd:token`’ datatype: leading and trailing whitespace is stripped; other sequences of whitespace are normalized to a single space character.

Each of the rules described in previous sections that uses a ‘`uri`’ attribute to specify a schema, can instead use a ‘`typeId`’ attribute to specify a type identifier. The type identifier can be associated with a URI using a ‘`typeId`’ element. For example,

    <locatingRules xmlns="http://thaiopensource.com/ns/locating-rules/1.0">
      <namespace ns="http://www.w3.org/1999/xhtml" typeId="XHTML"/>
      <typeId id="XHTML" typeId="XHTML Strict"/>
      <typeId id="XHTML Strict" uri="xhtml-strict.rnc"/>
      <typeId id="XHTML Transitional" uri="xhtml-transitional.rnc"/>
    </locatingRules>

declares three type identifiers ‘`XHTML`’ (representing the default variant of XHTML to be used), ‘`XHTML Strict`’ and ‘`XHTML Transitional`’. Such a schema locating file would use ‘`xhtml-strict.rnc`’ for a document whose namespace is ‘`http://www.w3.org/1999/xhtml`’. But it is considerably more flexible than a schema locating file that simply specified

    <namespace ns="http://www.w3.org/1999/xhtml" uri="xhtml-strict.rnc"/>

A user can easily use `C-c C-s C-t` to select between XHTML Strict and XHTML Transitional. Also, a user can easily add a catalog

    <locatingRules xmlns="http://thaiopensource.com/ns/locating-rules/1.0">
      <typeId id="XHTML" typeId="XHTML Transitional"/>
    </locatingRules>

that makes the default variant of XHTML be XHTML Transitional.

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