WRITEMODBUS command uses Modbus function 16 to write to the slave 16 bit at a time. So it depends on what data you write to the slave it can be 0 or 1 at any bit position. Remember that it is not setting a single bit, but setting 16-bit.
E.g. if the slave is another FMD or Nano-10 PLC and you WRITEMODBUS to the register 16 it will set outputs #1 to #16 simultaneously. However notes that any output that is under control of the PLC program will only be changed for one scan time and the next I/O update it will be set back to the ON/OFF value computed by the PLC program.
