I know, at times, the Client do not have a policy to give out letters for Contractors
So, it can be tricky to get, almost impossible. In that case, just submit the prime vendor-Client relationship letter.

Make sure that you have your email conversations, your cube photos,your batch and everything that proves you that you are currently working with the client...


But coming back to your questions, its THE most strongest doc in H1 visa...if you have it, its gold....But, does not mean, its mandatory......