請教xiaobaitu和Martinlu等大俠, H1 TRANSFER的問題

以前大家都說要收到USCIS的RECEIPT後才能去新單位上班...可俺這個新單位的律師卻告我說隻要收到律師寄往CIS的FEDEX的DELIVERY RECEIPT後俺就能上班了....原信摘抄如下
....we can confirm that Fed Ex delivered the petition to the immigration service today so he can start work anytime or give notice as the case may be. We will send you an I-9 instruction letter so you will know how to complete the I-9 form...

俺徹底暈了...到底哪個是最新規定...還有I9是啥東東...最沒聽壇裏的大俠門提過

多謝先了