When you email HR for an experience letter, put the essentials in the first two lines — full name, employee ID, designation and exact employment dates. HR processes these in bulk; a request they can action without replying for details gets answered fastest.
An experience letter confirms what you did and for how long. A relieving letter confirms that you completed your notice period and were formally released. New employers in India and the Gulf frequently ask for both.
Practice varies by jurisdiction. Many employers issue one as a matter of policy, and some labour laws require a service certificate on request. If they refuse, offer letters, payslips and tax records are usually accepted as alternative proof.
HR or the people operations team, with your reporting manager copied. Managers rarely have the authority to issue letterhead documents themselves.