Relieving Letter vs. Service Certificate
A relieving letter serves as evidence of an employee's departure from duties. It benefits the employer to provide a relieving letter to an employee. A relieving letter does not reflect any service history nor certify the skills or abilities of an employee. Therefore, you can issue a relieving letter even if the employee has not gained any experience.
On the other hand, a service certificate is issued to certify the service history and the abilities an employee possesses or has gained by working in your organization. You need not issue a service certificate if you feel that the employee does not possess any skills to showcase, but you can issue a relieving letter.
The former is addressed to "whomsoever it may concern," whereas the latter is addressed to the person/employee concerned. The latter will be worded like, "Accepting your resignation dated, you are hereby relieved from service." No comments about the employee's service or designation are necessary (you can just mention their name and employee number), and it is not necessary to issue it on company letterhead. It can be issued on an official correspondence pad used to communicate with departments and employees.