Reliable messaging (or reliable send) guarantees that the order of sender sending and receiver receiving the message is equal. For instance, if sender sends message in order of A,B,C,D,E then receiver receives them in the same order of A,B,C,D,E without data being broken.
Reliable messaging is advantageous for its high reliability but relatively slower than Unreliable Messaging in transmitting time.
Reference: Protocol type of ProudNet