Q Series?
it has been a few years since i did any Mitsi programming and my memory isn't what it was (my hairline either)
SM403 from memory is an 'always on' bit
[MOV K3X0 Z0]
MOV = Move
K3X0 = Read 12 bits starting at Input address X0
K1 = 4 bits,K2 = 8 bits,K3 = 12 bits,K4 = 16 bits when used with this type of instruction.
Z0 = Index Register 0
i've used Z registers in the past for indirect addressing.
[>= K9 D126] etc = if the D register is greater than/equal to 9
MOVP = Move with rising edge pulse
The other instructions i haven't used however if you go into the instruction detail (double click it) you should see a brief explanation of what each instruction does.
I don't have GX developer installed currently but hopefully what i've wrote should get you started.