This is a basic API to develop extensions which connect to different payment transaction gateways.
The extension transactor has the purpose to enable online payments connecting to various gateways.
You must configure it in the Extension Configuration of the Settings backend module. Set the compatibility mode (basic.compatibility) to 0 for the new API. Older extension versions might need the old API and a 1 here.
Since version 0.9.0 the transactor extension contains a middleware, which payment extensions can use for a gateway instant message after the payment.
Use the transactor parameter as transactor=mygateway in the backend url to your payment extension page.
Your extension`s ext_localconf.php file must configure the middleware in TYPO3 10+:
$GLOBALS['TYPO3_CONF_VARS']['FE']['transactor_include']['mygateway'] = \Foo\Bar\Controller\ServerResponseController::class . '::processRequest';
Then your class ServerResponseController can receive the calls from the instant messages from your gateway.