Need for client letter document

[mprecd]

Hello,
I am working at client place on Employer-vendor-mainVendor-Client model. I have my H1 renewal due in Oct. My client is ready to provide client letter but wants to see "the copy of the requirement from the US citizenship and Immigration Services that indicates the need and purpose of a client letter ". Can anyone please redirect me where can I find this document.

Thanks and really appreciate any guidance in this regard.