Relieving letter suggests that you have handed over all the documents and items of the organization. There is nothing that you owe to the organization. It talks about detachment between employer and employee. It also suggests that the employer or employee have agreed to scrap their relation.
On the other hand experience letter talks about duration of your stay in the company and your last designation.
These days, when you move from one company to another they ask for Relieving Letter or Resignation Acceptance Letter to be ensured that you have actually left the organization. In good olden days…people use to take leave from their current employer…go and work with prospective employer…learn about the culture and environment and then suddenly disappear.
So, asking for relieving letter is just to trash such practice in the corporate world.
I hope this will help.
Regards
Sanjeev Sharma
(BLOG:
http://sanjeevhimachali.blogspot.com/)