Trying to migrate user data into multi value term reference field
Use a callback with a separator to transform the source value to the destination value. For example:

  $this-&gt;addFieldMapping('statuut_nl', 'stat_id')
   -&gt;separator(';')
   -&gt;callbacks(array($this, 'computeValue'));

  protected function computeValue($value) {
    $allowed_values = array(
      2 =&gt; 'Postdoctoraal onderzoeker',
      ...
    );

    return ($allowed_values[$value]);
  }