A-Number, Alien Registration Number in full, is an eight- or nine-digit number generated and assigned by USCIS to noncitizens when they apply for permanent resident status in the US. You can find your Alien Registration Number in the following documents: green card, immigrant visa, immigrant data summary, employment authorization document and some other documents. When filing out I-751 petition, remember, if your A-Number is fewer than nine digits, insert a zero after the “A” and before the first digit to create a nine-digit number.
Meanwhile, some categories of applicants may not have A-Number. That is why “if any” stands in brackets near every A-Number reference in Form i-751. So, if this question does not apply to you type or print “N/A,” unless otherwise directed.