Look like that info is removed from the latest web/pdf documentation
yes they are call merchant defined fields.
From http://www.authorize.net/support/merchant/Integration_Settings/Additional_Integration_Features.htm
You may also choose to submit merchant-defined fields to further customize the information that is included with a transaction. Merchant-defined fields are any fields that are not recognized by the payment gateway as standard application programming interface (API) payment form fields. For example, you may want to provide a field in your checkout process where customers may provide specific shipping instructions or product color information.
Merchant-defined fields are included with the transaction response and in the merchant confirmation email for the merchantโs records. However, they are not provided on the Transaction Detail page in the Merchant Interface. Contact your Web developer for more information on how to submit merchant-defined fields with transactions to the payment gateway.
For DPM, just added to the one of the fields post to authorize.net and it will echo back to you on relay response.
