In general, if the RFE states Right to Control or Employer-Employee relationship, USCIS is basically requesting letters from all parties involved (end client letter, end client contracts, prime/main vendor, mid-vendors etc) documenting their role.
Below is a snippet of the RFE issued by USCIS recently for RIGHT TO CONTROL/EMPLOYER-EMPLOYEE RELATIONSHIP listing the documents needed (All confidential and identifying information has been redacted)
Below is a snippet of an RFE where an end-client letter though included was rejected as proper evidence because the client letter did not contain all the elements required in the client letter (on a high level it is recommended the end-client letter contain the name of the employer, duration of the assignment, contractual path, job duties and minimum educational requirements for the job. Further, it has to be signed by the authorized signatory of the end client with complete contact information. Lately, we are starting to see an increase in the number of RFE’s requesting end client contracts and purchase orders even when an end-client letter is included.
Practical Issues: Employers (IT Consulting based companies/3rd party worksite-based employers) should try to avoid placing their H-1B employees for assignment at those end clients who refuse to give verification letters for H-1B based contractors or refuse to release such information for USCIS verifications since USCIS has mandated a need for such documents as a part of the H-1B petitions. Further employers through their attorneys wherever appropriate and allowed should reach out to appropriate authorized officials of various parties involved and discuss the feasibility of getting such end client documents required for H-1B adjudications by signing a Non-Disclosure or Confidentiality Agreement. By signing a Confidentiality or Non-Disclosure agreement attorneys can try to limit the usage of such documents for the sole purpose of using it to provide to USCIS. A few end clients are comfortable in providing such documents after such non-disclosure agreements are signed. Alternatively, the contracts and purchase orders of end client can also be requested after any sensitive information (such as RATE etc) is redacted as a few end clients are comfortable releasing such documents partially after redacting sensitive information or after a Non-Disclosure agreement is signed. USCIS based on its Feb 22, 2018 memo does require that availability of non-speculative work is proven through end client documents and providing vendor letters or implementation partner letters does not satisfy the regulatory requirement.