If you look at the sdlxliff schema it has "cmt-meta-defs" under "doc-info":

  <xsd:element name="doc-info">
    <xsd:complexType>
      <xsd:all>
        <xsd:element minOccurs="0" maxOccurs="1" ref="sdl:rep-defs" />
        <xsd:element minOccurs="0" maxOccurs="1" ref="sdl:cmt-defs" />
        <xsd:element minOccurs="0" maxOccurs="1" ref="sdl:cmt-meta-defs" />　<---------- This one
        <xsd:element minOccurs="0" maxOccurs="1" ref="sdl:rev-defs" />
      </xsd:all>
    </xsd:complexType>
  </xsd:element>

However, I have never seen this element in an actual sdlxliff file.

Aslo the SDL SDK has "MetaData" member of IComment:

Question, how do you add metadata to a comment in Trados? And what is this metada used for?