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A表、B表是怎麽回事？

-If your priority date is earlier than the cut-off date in A Form, then that means the visa number is available to your case. Thus, you can file I-485 or your I-485 may be approved (if you have a pending I-485); If your priority date is earlier than the cut-off date in B Form, then National Visa Center may ask you to provide the documents and you can file DS 260 to apply for the immigrant visa outside US; sometimes you may also can file I-485 within US using B Form if you are present in US.

Whether you can file I-485 based B Form, you need to check this website each month:

Adjustment of Status Filing Charts from the Visa Bulletin | USCIS

如果B表 current, A表還沒有，可以做什麽？

-See above link to see whether you can file I-485 if you are present in US.