Hi, there is no such requirement. By signing the copy of the increment letter, the employee confirms his/her acceptance of the revised salary, and the same can be considered as an acceptance letter. If you try to collect satisfactory increment letters from employees, you might face hardship as satisfying 100% will not be possible since self-contentment will not be there with all employees.
Current Employee Expectations
Nowadays, employees expect increments based on their volume of loan commitments rather than their performance.