Example of how to show a field at checkout for addon packages only.

addon-package-fields.php

<?php
/*
  This field is for addon packages only.
*/
if(!empty($_REQUEST['ap']))           //might want to also look up ap to see if it's active/etc
  $profile = true;
else
  $profile = 'only_admin';
  
$fields[] = new PMProRH_Field(
        "student",              // input name, will also be used as meta key
        "text",                 // type of field
        array(
		        "levels"=>array(2,3,4,5),
            "size"=>40,         // input size
            "class"=>"student", // custom class
            "profile"=>$profile,    // show in user profile
            "required"=>true,   // make this field required
			"memberslistcsv"=>true
	));