For example in PhoneCallInstall.php I see
[quote:rvrgjddf]
$fields = array(
array('name' => _M('Subject'), 'type'=>'text', 'required'=>true, 'param'=>'64', 'extra'=>false, 'visible'=>true, 'display_callback'=>array('CRM_PhoneCallCommon','display_subject')),
array('name' => _M('Contact Name'), 'type'=>'hidden', 'extra'=>false, 'visible'=>true, 'display_callback'=>array('CRM_PhoneCallCommon','display_contact_name')),
array('name' => _M('Phone Number'), 'type'=>'hidden', 'extra'=>false, 'visible'=>true, 'display_callback'=>array('CRM_PhoneCallCommon','display_phone_number')),
array('name' => _M('Customer'), 'type'=>'crm_company_contact', 'param'=>array('field_type'=>'select'), 'extra'=>false),
[/quote:rvrgjddf]