xsl:avt 

This type is used for all attributes that allow an attribute value template.
The general rules for the syntax of attribute value templates, and the specific
rules for each such attribute, are described in the XSLT 2.0 Recommendation.

Simple Type Information

Used By

Attributes xsl:analyze-string/@flags, xsl:analyze-string/@regex, xsl:attribute/@name, xsl:attribute/@namespace, xsl:attribute/@separator, xsl:element/@name, xsl:element/@namespace, xsl:message/@terminate, xsl:namespace/@name, xsl:number/@format, xsl:number/@grouping-separator, xsl:number/@grouping-size, xsl:number/@lang, xsl:number/@letter-value, xsl:number/@ordinal, xsl:processing-instruction/@name, xsl:result-document/@byte-order-mark, xsl:result-document/@cdata-section-elements, xsl:result-document/@doctype-public, xsl:result-document/@doctype-system, xsl:result-document/@encoding, xsl:result-document/@escape-uri-attributes, xsl:result-document/@format, xsl:result-document/@href, xsl:result-document/@include-content-type, xsl:result-document/@indent, xsl:result-document/@media-type, xsl:result-document/@method, xsl:result-document/@normalization-form, xsl:result-document/@omit-xml-declaration, xsl:result-document/@output-version, xsl:result-document/@standalone, xsl:result-document/@undeclare-prefixes, xsl:sequence-constructor-group/@byte-order-mark, xsl:sequence-constructor-group/@case-order, xsl:sequence-constructor-group/@cdata-section-elements, xsl:sequence-constructor-group/@collation, xsl:sequence-constructor-group/@data-type, xsl:sequence-constructor-group/@doctype-public, xsl:sequence-constructor-group/@doctype-system, xsl:sequence-constructor-group/@encoding, xsl:sequence-constructor-group/@escape-uri-attributes, xsl:sequence-constructor-group/@format, xsl:sequence-constructor-group/@href, xsl:sequence-constructor-group/@include-content-type, xsl:sequence-constructor-group/@indent, xsl:sequence-constructor-group/@lang, xsl:sequence-constructor-group/@media-type, xsl:sequence-constructor-group/@method, xsl:sequence-constructor-group/@namespace, xsl:sequence-constructor-group/@normalization-form, xsl:sequence-constructor-group/@omit-xml-declaration, xsl:sequence-constructor-group/@order, xsl:sequence-constructor-group/@output-version, xsl:sequence-constructor-group/@separator, xsl:sequence-constructor-group/@standalone, xsl:sequence-constructor-group/@terminate, xsl:sequence-constructor-group/@undeclare-prefixes, xsl:sort/@case-order, xsl:sort/@collation, xsl:sort/@data-type, xsl:sort/@lang, xsl:sort/@order, xsl:value-of/@separator

Source 

<xs:simpleType name="avt">
  <xs:annotation>
    <xs:documentation>This type is used for all attributes that allow an attribute value template. The general rules for the syntax of attribute value templates, and the specific rules for each such attribute, are described in the XSLT 2.0 Recommendation.</xs:documentation>
  </xs:annotation>
  <xs:restriction base="xs:string"/>
</xs:simpleType>