H1B Interview : What is Client letter ?

One of my friend have interview next week. in his case Client wont issue Client letter.

But the Employer has a Support letter which states his client, jobs, project salary etc.

(A letter which we used at the time of H1b filing)

is this same as client letter? can this be used when VO asks for Client letter?

It’s not a client letter as it is not on client letter head. He can still submit that letter along w/ explanation that client doesn’t issue letters as per their policy. Has the client informed about this policy in email? If yes, then a copy of that email will also come in handy.