The Reset_Process command is defined in Section 7.1 of the IEC-870-5-104 protocol specification document, the section covering Station Initialization. This command is used as part of the connection procedure to close an open socket connection.
On page 45 of the protocol specification, it describes the process as:
Figure 20 shows the controlling station establishing a connection by giving an Active Open call to its TCP. The controlling station (Master) then sends Reset_Process to the connected controlled station (Slave), which confirms back the Reset_Process and gives an active close call to its TCP.
Please refer to Figure 20 of the protocol specification document for a graph of the complete communication procedure using the Reset_Process command.