In Delhi, India, an immediate employer (Contractor) can indeed opt to use an "Employment Contract" instead of an "Appointment Letter" when hiring an employee. It is crucial to ensure that the contract clearly outlines all terms and conditions of employment to avoid any misunderstandings. Regarding the contract period, it is advisable to specify whether it aligns with the duration of the contract with the Principal Employer or if it is for a different timeframe. However, it is recommended to include a specific contract period to provide clarity and avoid ambiguity in the employment relationship. Ensure that the contract complies with all relevant labor laws and regulations in Delhi, India, to protect both the employer and the employee.