Thank you, I found the answer

来源: 于 2005-09-29 14:16:03 [旧帖] [给我悄悄话] 本文已被阅读：次

with website: http://www.immihelp.com/visas/i539.html

A#: A# number stands for eight-digit alien number. You will have A# number only if you filed certain immigration petitions such as I-485, EAD etc. Visitors don't have A# number and you can write 'None' in this field.