Here is an example of a unit element holding a unitRef attribute that points to a definition of the unit in the TEI header.
<measure> <num>3</num> <unit unitRef="#ell">ells</unit> </measure> <!-- In the TEI Header: --> <encodingDesc> <unitDecl> <unitDef xml:id="ell"> <label>ell</label> <placeName ref="#iceland"/> <desc>A unit of measure for cloth, roughly equivalent to 18 inches, or from an adult male’s elbow to the tip of the middle finger.</desc> </unitDef> </unitDecl> </encodingDesc>