Serin and serout

[BinaryBob0001]

Ok when I got in to this I wanted to know how everything works and so now I have a hard question: I think. How does the serin and serout statement work, I know it's a serial port command, but how do you decide what argument goes where? Like if I place something on the third argument, why does it go to the engines and not the relays? Where do I find out this stuff? Thanks.