In general, the two most common ways to obtain legal permanent residence in the United States (symbolized by the so-called “green card” are through certain family members or through employment.
If through employment a potential employer goes through the labor certification process to show, if able, that there are no U.S. workers who are ready, willing and able to be hired for the job.
There are several categories in which there are exceptions to having the potential employer go through this recruiting process such as Aliens of Extraordinary Ability, Outstanding Researchers and National Interest Waiver cases. No test of the labor market is needed for these categories, but the regulations and case law require proof that the elements of each are met.
In those cases that do not require an alien labor certification, letters of recommendation are a “must.” These letters are not in a style that one requests for a job, that is showing a person’s uniqueness, hard working style, or easy to work with personality. The letters that are needed are all about a petitioner’s IMPACT on a field of endeavor.
Therefore, the key is picking the field of endeavor as all letters of recommendation go toward this. I like to make the field as narrow as possible to show impact easier, but not so narrow that that there may be a problem finding an employer in the same field. An employer is not necessary to actually file the petition for Outstanding Researchers (EB 1-2) and National Interest Waivers (waiver of alien labor certification) (EB-2) but is necessary for Aliens of Extraordinary Ability EB 1-1. However, a potential employer is for all these employment-based categories once the initial petition is approved, to get the next step, the immigrant visa or if in the US the application for adjustment of status to legal permanent residence. Such employment must be in the field that the initial approved petition was approved for.
Once the field is picked the letters of recommendation go toward showing the alien’s impact on the field.
Letters should come from people that do not know the alien directly but do know if his/her work. USCIS still thinks the letters only from people who know the alien, maybe a colleague or mentor, will always be favorable and therefore not as reliable as compared to writers who do not work with the alien and, therefore, will give an unbiased impact opinion as he/she has “nothing” to lose.
Letters should come from inside the U.S. and if can, from outside the U.S. If in the U.S. they should come from as many different locations as possible to show wide impact. If can, letters should come form industry and well as form academia.
Since a number of writers of the letters are not used to writing in the way the USCIS favors, obtaining a rough draft is advisable in order to edit it. Also, sending sample letters to writers is helpful as a guide.