what should an offer letter contain?

While the exact offer letter format may differ across companies and industries, every job offer letter should contain the following for employees in India:
Position at company (job title), job description, start date, and probationary period. You should explain that an employee’s suitability for the job will be evaluated during their probationary period. In India, probationary periods vary among roles and industries, but typically last from one month to one year.
Worker classification information. Establish whether the new hire will be an employee or contractor.
Working hours. Outline the expected working hours and any overtime policies that the employee will be subject to. In India, the maximum working week is 48 hours, with no more than nine hours a day. While overtime rates vary for private sector employees, factory workers are entitled to double their regular pay rate for additional hours. For more on working hours and overtime, read our guide to hiring in India.
Compensations and benefits: Salary.  Specify the employee's salary or hourly compensation in INR, as well as any other compensation they may receive (equity compensation, bonuses, incentive pay, etc.). Specify that the employee’s salary will be paid monthly, per India’s standard.
Equity.  If applicable, specify any equity compensation they will receive. The most common types of equity compensation for employees in India are stock options, restricted stock units (RSUs), and Employee Stock Purchase Plans (ESPPs).
Benefits. Benefits could be outlined in the offer letter, but be sure to address them in general terms so that if they change in the future, an amendment to the offer letter isn't required. In India, the following benefits are mandatory for full-time employees (though not always listed in an offer letter): employee pension scheme contributions, employee provident fund, vacation entitlements, statutory holidays, sick leave, and maternity leave.
Vacation.  The employment contract should include details about your company's vacation leave policy, especially if you offer more time off than the typical 15 days of “earned leave” for private sector employees (the exact minimums in India vary between states). To learn more about leave and statutory holidays in India, read our guide to hiring in India.
Termination policy. Clearly explain the terms of termination, including the notice that will be provided, and any conditions that may lead to termination. Note that there are limited reasons an employee can be terminated involuntarily in India, and at-will employment does not exist. The notice period required by law can vary between states, but employers have to provide at least one month.
Confidentiality and non-disclosure.  Include in the offer of employment a clause outlining the employee's responsibilities regarding confidentiality and non-disclosure of the company's information. Non-disclosure agreements (NDAs) are considered legally binding in India, though they should be stamped and registered to ensure enforceability.