You should use this letter If the employer is not satisfied with the performance of the employee during the probationary period and wants to extend the probation period for some more months.
Probation period, otherwise known as trial period, is a period fixed for a new employee at the time of joining under a contractual arrangement during which both the employee and employer is exempted from certain contractual obligations, especially notice period for termination or quitting job. Generally, the probationary period is for a period of 3-6 months. During the period, the employer tests the performance of the employee. If satisfied, the employee is taken on roll at the end of the period, wherein the employee is given more job benefits and is bound by more contractual obligations.