The extension transactor has the purpose to enable online payments connecting to various gateways.
Some extensions might need the old API which has however been removed in version 0.11.0.
Since version 0.9.0 the transactor extension contains a middleware, which payment extensions can use for dealing with 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.