UnSafeDisc 2

I have a few questions about UnSafeDisc 2.
How do you know wich files to use it on?
Is there any downside of using Unsafedisc2 except that you dont get a perfect copy(1:1)?
You still can play the game dont u?And online?
And if i wanted to use USD 2 then how shall i use it?
Where can i find it?