感觉头晕了，EB1a申请需要多少材料，今天听说还要一个cover letter

EB1a申请需要多少材料: 这个网址上列出的是不是就够了 http://www.ezthelife.com/archives/3898

请问cover letter是什么，要写多少内容。

我查到的有的写的很多，有的很少内容，比如下面这个很少内容。

Response to RFE                                                                                                                      XX, Ph.D.
XXXXXXXX
XXXXXXXX
                                                                                                            Tel: [phone]   Fax: [phone]
                                                                                                     [e-mail]
January 5, 2015
Department of Homeland Security
USCIS Texas Service Center
P.O. Box 279030
Dallas, TX 75227-9030

RE: REQUEST FOR EVIDENCE for I-140 (EB-1a) (Premium Processing)
       Petitioner/ Beneficiary: Dr. XX

Dear Officer:
  I would like to express my sincere gratitude to you for carefully reviewing my case.  Please find enclosed following documents in supporting my immigration petition:

1. Response Letter (detailed list of supporting documents enclosed).
2. Documentation at corresponding exhibits demonstrating my original and significant contribution (Please see list of INDEX OF EXHIBITS for details).
3. Certifications of translation.

Thank you for your consideration of this petition. If you have any questions or need any additional information, please feel free to contact me at my phone number.

Respectfully submitted,

XX. Ph.D.

Enclosures: Index of Exhibition