Opening/closing years

No it does not refer to a certain number of years. The bases of assessment in opening years simply show how we go from an actual basis in the first tax year to the use of CYB at the earliest available opportunity - this may be the 2nd, 3rd or even 4th tax year! In closing years we use CYB through to the penultimate tax year but then need to know what will be assessed in the final tax year.