虛心請教485中immigrant visa No如何得到?

關於immigrant visa number: 在http://uscis.gov/graphics/howdoi/immvisa.htm#apply上沒講清楚.http://uscis.gov/graphics/services/residency/employment.htm上講:the State Department must give the applicant an immigrant visa number, even if the applicant is already in the United States. When the applicant receives an immigrant visa number, it means that an immigrant visa has been assigned to the applicant.
所以,在遞交485前(EB-1a 140已批,through employment)還沒有immigrant visa number,移民局在485收據上給你此immigrant visa number.不知我理解的對不對?特此虛心請教.
如果不對,如何申請此immigrant visa number? 請詳細說明.
多謝各位.