Recently we removed x_response_subcode from our SIM, Relay and Silent Post responses. The data returned in x_response_subcode was for internal tracking only. I apologize if this caused any issues with your integration.
As a best practice, we recommend that when you parse responses for SIM, Relay Response and Silent Post, that you parse the data for name/value pairs rather than parsing for the order that the fields are usually returned in. We do not document the field order for these integration methods and it is subject to change without notice. Parsing our response by field name will prevent errors in your integration should this occur.