Un-Acknowledged (UnAcked): the message is sent on the network as required with no return acknowledgement that it was received.
Un-Acknowledged Repeat (UnAckedR): this service sends out the same message a fixed number of times without dealing with feedback from every device.
Acknowledged Service with Automatic Retries (Acked): this is the default message type. When a message goes out as Acked service, then the receiving device sends an acknowledgement (Ack) back to indicate that the message was received correctly. If an Ack is not received in a given time frame then the protocol automatically resends the message.
Request/Response: application to application message with the response returning data with it.
Message services are not coded as part of your application but stored separately in the Network Image. This means at installation time you can determine which delivery method is most appropriate. Or at some later time you can come back and change the delivery method without changing your application code.