Thanks Pijing,one more question?回复：1. New employment - 回复：H1-B C

In Part two,item 4,
Prior Petition. If the beneficiary is in the U.S. as a nonimmigrant and is applying to
change and/or extend his or her status, give the prior petition or application receipt #:

In your Blog,the answer is "4. Prior Petition.
如果你在境内有过 EOS/COS，请填其收据号。"
I am very confused about the answer. What is "EOS/COS"? Can I use my I-485 A#? Please give me some explaination.