回复：回复：回复：回复：Alien # vs. Immigration visa #

According to I-485 document introduction, it mentions that:
WHo may file:
Based on an immigration petition. You may apply to adjust of status if:
1. an immigration visa number is immediately available to you based on an approved immigration petition; or
2. you are filing for ....

I am not sure about what this Immigration visa number is? However, when I filled in the I-485 form, it asked me for A#, that is why I am really confused.

Thansk a lot.