Add some Register Helper field data to the order notes at checkout.

add_rh_fields_to_order_notes_via_pmpro_checkout_order.php

/**
 * Add some Register Helper field data to the order notes at checkout.
 * 
 * 1. Update the $fields_to_add array below with the fields to save into the order notes.
 * 2. Make sure PMPro Register Helper is active, the fields you want are setup and being added to the checkout page form.
 * 3. Add this code to a custom plugin.
 *
 * Note that some advanced field types may not store their data in a human readable format.
 * You may want to update this function to handle those field types specifically or to use
 * nicer labels in the notes/etc.
 */
function add_rh_fields_to_order_notes_via_pmpro_checkout_order( $order ) {
	$fields_to_add = array( 'agreement' );
	foreach( $fields_to_add as $field ) {
		if( isset( $_REQUEST[$field]) ) {
			$value = sanitize_text_field( $_REQUEST[$field] );
			if( !isset( $order->notes ) ) {
				$order->notes = $field . ': ' . $value . "\n";
			} else {
				$order->notes .= $field . ': ' . $value . "\n";
			}
		}
	}
	return $order;
}
add_filter( 'pmpro_checkout_order', 'add_rh_fields_to_order_notes_via_pmpro_checkout_order' );