Rules parameter description to appear in different lines
This should work

 $descp = "line1 &lt;/br&gt; line2";

 $items['my_custom_action'] = array(
    'label' =&gt; t('Some Label'),
    'triggers' =&gt; array('any'),
    'named parameter' =&gt; TRUE,
    'parameter' =&gt; array(
      'par1' =&gt; array(
        'type' =&gt; 'list&lt;text&gt;',
        'label' =&gt; t('Select an option'))),
        'options list' =&gt; array('1' =&gt; t('Option1'), '2' =&gt; t('Option2')),
        'description' =&gt; t($descp),
        'restriction' =&gt; 'input',
      ),
    ),
    'group' =&gt; t('My own group'),
  );