The merchant can send some extra parameters in the hidden fields of the order form, in order to retrieve them as feedback parameters after the payment.
One of the hidden fields is the PARAMPLUS parameter. This field submits some parameters and their values you would like returned in the feedback request (as a querystring).
Be careful when you generate your SHA-OUT hash based on the incoming feedback parameters. You have to exclude all the extra parameters, you sent with the PARAMPLUS parameter, before generating the hash.