RTN stands for Register Transfer Notation (RTN) and also called as Register Transfer Language (RTL) The significance of RTN is that symbolic notation used to describe the behavior of micro operations.

We use the notation M[X] to indicate the actual data stored at location X in memory, and ← to indicate a transfer of information. In reality, a transfer from one register to another always involves a transfer onto the bus from the source register, and then a transfer off the bus into the destination register. However, for the sake of clarity, we do not include these bus transfers, assuming that you understand that the bus must be used for data transfer.

Now it presents the register transfer notation for each of the instructions in the ISA for MARIE.