Function 0x06 "Write Single Register" writes a single 16-bit Holding Register on a Modbus slave, with both the address and the value encoded on 16 bits each in the request.

The 0x06 request format is: function code on 1 byte (0x06), register address on 2 bytes (0-65535), and register value on 2 bytes (0-65535) -- 5 bytes of PDU plus the transport headers. The dual function code is 0x10 "Write Multiple Registers", which writes several consecutive registers in a single frame, up to a limit of 123 registers per request.

Prefer 0x10 over a loop of 0x06 calls whenever you have to write more than two related registers: it is atomic at the bus level and easier for the slave to handle.