Thank you for the answers and references you provide 

I just wanted to learn to mine.
I already have asic S9 2 units.
At the time will start mining and read the guide on some POOL. I am still a little confused for the difficulty selection.
Is there a difference in the selection of Low difficulty or High difficulty?
With the tools I have, which difficulty should I choose?
And what is the recommended standard difficulty for my asic?