This notice lets you ask your employer for a relieving letter after you quit your Job and have either served or bought out the notice period(if any).
You need it in case your former employer have not yet provided you the relieving letter even after you are done with all the required formalities of the company.
To make this notice legally enforceable, it must be send by a lawyer hired by the concerned person. Note that notice must be signed by the lawyer on his/her letterhead.
You may use it as a former employee of company if you are seeking the relieving letter after having served or buying out the notice period.