Visa authorities usually require the experience letter on company letterhead, signed, stamped, and stating your job title, dates, salary and duties. Say this explicitly in your request — a generic letter that omits salary or duties often gets rejected and has to be re-issued.
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.