Thank you all
spcomputing, I agree with you about the multilayer pcb in a pdf file, in which you can't select a layer anyway so what's the point! I tried to convert that pdf into a pcb file, but I can't find anything about it (tried "pstoedit" but no luck). But Chagrin is right, both 5v pins are connected together, and same for the GND pins.
Retrolefty I agree with dhenry, from what I understand the 12VDC go into the LM2576 which then distribute 5VDC to opto isolators, then to those 5V pins. I just wasn't sure about myself so I asked..I always ask when in doubt! And yes, I knew about converting the 16 analog to digital, we already discussed that in another topic of myself, I want to save pins because my project will be quite big in the future! Of course I will first try to connect some relays directly to the arduino, and once I get this working, only then I will try introduce the shift registers (buyed 5 of them so I'm ready to sacrifice some if things go bad
About the opto isolator, I've read that they are model 817C, and require something like 3mA each. Again, I'm not sure, but I guess I will just have to try and see. I just hope I will not fry the arduino
I think I'm now ready to buy this board and have lot of fun - or problems - with it!
Thanks again everyone, appreciated.