H1-B back ground check - only for employment? - Thank you!!!!!

I'm going back to China soon, and will need to revalidate my H1-B. I understand that I might be selected for a background check, but I don't know what kind of "background"? Only employment background on my employer? Or will they also do a criminal background check on me?

Another quesiton - When I re-validate my H1-B, are they strict on I-20 stuff? (I came to US as a student) because I lost the original I-20, and I don't have tran from one of the school I attended... (not a good student...)

THANK YOU FOR YOUR HELP!!!!!!