extensionsType 

You can add extend GPX by adding your own elements from another schema here.

Complex Type Information

Model

ANY element from ANY namespace OTHER than 'http://www.topografix.com/GPX/1/1'

Used By

Elements gpxType/extensions, metadataType/extensions, rteType/extensions, trkType/extensions, trksegType/extensions, wptType/extensions

Source 

<xsd:complexType name="extensionsType">
  <xsd:annotation>
    <xsd:documentation>You can add extend GPX by adding your own elements from another schema here.</xsd:documentation>
  </xsd:annotation>
  <xsd:sequence>
    <xsd:any namespace="##other" processContents="lax" minOccurs="0" maxOccurs="unbounded">
      <xsd:annotation>
        <xsd:documentation>You can add extend GPX by adding your own elements from another schema here.</xsd:documentation>
      </xsd:annotation>
    </xsd:any>
  </xsd:sequence>
</xsd:complexType>