Message Types are utilized by CU*BASE to determine what action is to be taken for this transaction.  The original message type is the incoming message received by CU*BASE from your vendor.  Depending on the Message Type received by CU*BASE, the system could perform any of the following type of transactions (this is not an all inclusive listing):

Balance Inquiry

Request for Authorization of Transaction

Advice of Authorization (Vendor/VISA/MC approved)

Transfer of Funds Request

Reversal of Transaction

Adjustment to original transaction

Completion of orignal transaction authorized

The Last Message Type is usually CU*Answers response to the original or subsequent messages sent by the vendor.  CU*BASE software is required to respond to the original message within a certain number of seconds back to your vendor and the response sent by CU*BASE is usually in what you will see in the Last Message Type field.