What kind of additional documents do USCIS normally ask when they send an RFE for I-140?

It depends on what document is missing or insufficient. Its a request to view the document that has failed to satisfy USCIS to adjudicate your case.

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what are the documents that have been asked in recent and most cases?

This is for I-140 on EB2 case.

Most of the time, regardless of your filing category, when RFE is issued under the employment based I-140, it can be seen as employer ability-to-pay the proofed salary, your education requirements to fulfill the job, your previous past experiences. Especially Nebraska Center was notorious in issuing RFE to the applicants who filed using his/her foreign degree credentials. Recently they have relaxed a little bit. Employer ability-to-pay has also been seen in recent days.

I am not sure why people get so nervous when they see RFE. To be honest with you, when they send you RFE, one should be happy to know that USCIS is NOW working on his/her petition which is a good sign, as considering 95,000 I-140 backlogs which was reported a month ago. This figure must have gone high as they have halted the premium service indefinitely.

laborperm@gmail.com

Regrads,
LaborPerm
laborperm.blogspot.com

So, what are the documents required in order to show employer ability-to-pay the proofed salary?

You should consult your lawyer if you receive such document. I don't know what is mentioned on your LC, and what you are making. They might ask for companys tax return as well. Talk to your lawyer once you see it.

OK, thanks a lot for your help; I'll keep you posted on this.

Let me know if you need any further assistance. Good luck.

Regards,
LaborPerm
laborperm@gmail.com