I have had 3 employers (2 paid full time, 1 volunteer part time) during my OPT. I have reported to my school about each of my employers within 10 days of starting employment with each of these and my international student adviser has updated them in the SEVIS system accordingly. I am applying for my H1B visa. I am not sure if I am required to provide proof of employment from both of my past employers but I would rather err on the safe side. I have an I-9 form for both the full time employers (current employer is a full time employer and is sponsoring my H1B) However, since 1 of my employments was a volunteer position, there is no I-9 available for it. Does USCIS require tangible proof of employment for this position or is only the current pay stub enough ?