A page's PHP already produces its data, so the same page can answer with that data instead of rendering its template. The PHP names what may leave the server in $padExpose - nothing else ever does, and an engine variable never does:

<?php

  $orders    = [ [ 'id' => 1, 'customer' => 'Ann', 'amount' => 12.5 ],
                 [ 'id' => 2, 'customer' => 'Bob', 'amount' => 7    ] ];
  $total     = 19.5;
  $padExpose = [ 'orders', 'total' ];

?>

<ul>
  {orders}<li>{$customer}: {$amount}</li>{/orders}
</ul>
<p>Total: {$total}</p>

Asked with &padFormat=json - or by a client sending Accept: application/json - that page answers the named variables as one JSON object. &padFormat=csv, or Accept: text/csv, answers the first list among them as a table with a header row. The template does not run for either:

<?php

  $json = padCurl ( $padGoExt . 'fragments/data_answers_1&padFormat=json' ) ['data'];
  $csv  = padCurl ( $padGoExt . 'fragments/data_answers_1&padFormat=csv'  ) ['data'];

?>

{$json}
{$csv}

{
    "orders": [
        {
            "id": 1,
            "customer": "Ann",
            "amount": 12.5
        },
        {
            "id": 2,
            "customer": "Bob",
            "amount": 7
        }
    ],
    "total": 19.5
}

id,customer,amount
1,Ann,12.5
2,Bob,7

• A page that exposes nothing renders its HTML as always. Asked outright with padFormat=, it answers 406 Not Acceptable - as does a format that does not exist, or CSV from a page with no list.
• $padOutputType = 'json' (or 'csv') in _config/config.php makes every page of an application answer its data; {output 'json'} does it from a template.
• A script or a React component fetches from the ordinary page: fetch('?orders&padFormat=json').