ModbusTCP “slave” and “server” mean the same thing. “Master/Slave” are older terms that have been largely replaced in many industries in favor of “Client/Server”.
Holding Registers are read-only from the point of view of the server (i.e. plc in this case) unless you check the “writeable” box in the server configuration (shown in purple below).
Holding registers and Input registers are from the point of view of the client. The client normally writes holding registers and reads input registers.
Care must be taken when using “writeable” holding registers due to the cyclic nature of plc tasks, so that any data written by the HMI is not immediately overwritten by the plc on the next cycle.