Hello,
Many companies have different procedures; however, I would like to address the legality of these two letters.
Let us start with the first and foremost issue of any employment... an employment contract between the employer and the employee.
When an offer letter is issued with all terms covered, including the date of joining and specifying that the letter shall be returned within a stipulated period, it is the first step of the contract.
Then, the employee signs the letter, hands it over to the employer, and states that they will join within the stipulated time; the contractual obligation begins for both parties. In fact, the employer cannot withdraw the offer, and the employee cannot avoid joining. If an employee does not join, legally, the employer can sue them for breach of contract.
An offer letter duly agreed upon by the employee completes the process of the contract, and hence there is no need for an appointment letter.
Consider if an offer letter does not mention the notice period, and the appointment letter discusses it, then the employee can argue that they were not informed about the notice period before resigning from their previous job, and hence cannot accept it. This plea would be upheld by any court.
Therefore, it is essential that offer letters are comprehensive, and the organization must ensure they are duly signed.
If this process is in place, then there is no need for an appointment letter. In fact, I do not recommend having two letters, as the terms in the appointment letters can be challenged.
Siva