Hi, Appointment/Offer letters are employer-specific. There is no prescribed standard format for this.
Resignation letter
A resignation letter is employee-specific. Through a resignation letter, the employee communicates officially to the employer that he/she is not interested in continuing with the organization anymore. There is no standard format for this either.
While some members may share templates, it wouldn't be prudent to blindly follow them without understanding the significance of those terms and conditions.