How can I set frequency of each content type with the XML sitemap module?
As far as I can see, the priority form field is added to the content type form from xmlsitemap_add_link_bundle_settings(), which is called by xmlsitemap_node_form_node_type_form_alter().

The first function, in the development snapshot, is using the following form field definitions.

  $form['xmlsitemap']['status'] = array(
    '#type' =&gt; 'select',
    '#title' =&gt; t('Inclusion'),
    '#options' =&gt; xmlsitemap_get_status_options(),
    '#default_value' =&gt; $bundle_info['status'],
  );

  $form['xmlsitemap']['priority'] = array(
    '#type' =&gt; 'select',
    '#title' =&gt; t('Default priority'),
    '#options' =&gt; xmlsitemap_get_priority_options(),
    '#default_value' =&gt; $bundle_info['priority'],
    '#states' =&gt; array(
      'invisible' =&gt; array(
        'select[name="xmlsitemap[status]"]' =&gt; array('value' =&gt; '0'),
      ),
    ),
  );


The form field for the priority is invisible when the "status" field is set to 0.