installments during the year for taxes. In other words your employer is making you to pay 12 installments per year for your income taxes, it's voluntarily submitted with your consent, but it's not required by law.
Your employer is exploiting you for its own cash flow needs.
T1D is a the consent form you file to indicate that you prefer to have taxes deducted off your pay. Your employer does the T1D for you without you knowing because most people doesn't know much about taxes so it's common practice to submit taxes monthly and you are being exploited