They refer to employee resignation and state your request for release has been accepted wef -----date.
Just follow procedure for resignation of the organisation you work for ina clear documented manner.
Your mails should be marked to concerned team leader,HR department
Refer to the clause for exit in your offer letter.
something like this
I hereby submit request for voluntary exit wef ---- in accordance with para-- of my offer letter.
Polite but clear letter is all that is needed
2nd December 2017 From India, Pune