custom-ajax.class.php is designed to handle Ajax calls required to register at the frontend and backend at the same time. Its location is /mvc/custom-ajax.class.php. You can overwrite it in themes, or edit it directly if you integrate the Simpods Basic theme with a third party child theme.
No Simpods models are available for custom-ajax.class.php. If you want to use some functionalities provided by Simpods, you will have to include the classes you want.
If the class name is Custom_Ajax, it will be auto-instantiated.