%IW are input registers (analog)
%IX are discrete input registers. (digital)
They are both read only.
They are read with different function codes when polled using modbus TCP.
Function Code - Function
01 - READ COIL STATUS
02 - READ INPUT STATUS
03 - READ HOLDING REGISTERS
04 - READ INPUT REGISTERS
05 - WRITE SINGLE COIL
06 - WRITE SINGLE REGISTER
15 - WRITE MULTIPLE COILS
16 - WRITE MULTIPLE REGISTERS
Function codes 2 and 4.
Coils and Holding Registers are read using 1 and 3.
They are read/write, so you can write to them using function codes 5, 6, 15 and 16.
Thiago can confirm, but that is why they are not held in the same space.