Employers will usetax code 0t when they lack sufficient information about HMRC to derive a normal tax code. It tends to imply that no personal allowance is being claimed which is why everything is taxable under the basic, higher or additional rates. This code may be issued when you join a new job without giving a P45 or a starter checklist is not completed. It has the risk of paying too much tax in the beginning but it is then refunded to the individual when the HMRC corrects its books.