You can say something like the support letter for your GC application
category is intended for individuals who have made contributions of major
significance to their field and are recognized internationally as leaders in
their field. USCIS's requirement for that would be "one of a small
percentage of persons who have risen to the top of their field of endeavor"
in a particular field.
But I would assume many people in US or foreign country already knew/wrote
such letters before, and they know how it should look like.