An Income tax (IT) declaration is a declaration of an employee's proposed investments for receiving the maximum tax benefits.
The employee makes the declaration at the beginning of the financial year. New employees do it when they join a new company. The tax is calculated based on the declaration and regime selected by the employee.
Usually, the employer keeps the IT declaration window open for this purpose. Income Tax declaration also helps the employer estimate an employee's Income Tax (IT) for the current financial year. This deduction is called TDS or Tax Deduction at Source.