This is the tax form you will Apply, Register & Get for your corporation.  If you don't  know it, don't worry.

If you entered "1120S" after the corporation

must Apply, Register & Get Form 2553 no later than the 15th day of the 3rd month of  the tax year the election is to take effect.

Until Form (disregarded entities owned by a corporation enter the 2553 has been received and approved, )