Contains a code assigned to a merchant location. Merchants must use the First Data-assigned Merchant ID. The Merchant ID does not
conform to the standard definition of alphanumeric fields: it is right-justified and zero-filled on the left.
On a 0100 VRU Authorization only, the Merchant ID can be the First Data internal number or a merchant external number.
On a 0400 reversal message, this field must match the original 0100/0200 request.