I DIY NIW I-140 and replied RFE myself and finally got my I-140 approved in NSC. Hope this note will help you clarify your confusion about pettition letter, personal statement or cover letter.
所谓Petition Letter，有人理解是personal statement 性质的， 但在律师哪里一般就是给写的那封很长的cover letter。
移民官一打开你的材料，将先看到的是长达十多页的充满溢美之辞的信，感觉将会不同。一般移民官花在每个case上的时间也就是十五到二十分钟，而我们的材料一般来说都会有百八十页。移民官就是有天大的本事，他们也不可能全部看完。所以我们的Petition Letter/personal statement/Cover letter (whatever you call it) 一定要写的条理清楚，要让移民官知道你是作啥的，为什么你能在这方面作出成就，为什么缺了你就不转了。
我想大家都听说过"NY DOT Regulation", 如果没听说过，DIY-ers 还得下功夫考古一下。这个案例已经成为移民官衡量你的case标准。If you receive RFE, you will seeNYDOT at the very beginning of the RFE letter. 案例当事人是一位civil engineer at NY department of transportation. 很好的和"NY DOT Regulation" 相比较，突出为什么他不行，而你就可以。你在哪些方面与他不同。
“(A)n alien seeking a national interest waiver must prove that he or she will serve the national interest to a substantially greater degree than would an available U.S. worker having the same minimum qualifications. This is a rather onerous requirement. The alien cannot simply play an important role in a project which significantly benefits the field of endeavor; he or she must personally present a significant benefit to the field of endeavor.”做到这一点，成功的希望非常大了。
来源：未名空间（
www.mitbbs.cn）