BR: You have a second job or pension that is taxed at 20 per cent.
C: You pay the rate of income tax in Wales.
D0: Income from this source is taxed at the higher rate: 40 per cent.
D1: Income from this source is taxed at 45 per cent.
L: You are entitled to the personal tax-free allowance of £12,500 and no more.
K: You have a negative amount of personal allowance, possibly because of other income, taxable benefits from your employer and money you owe HMRC.
M: Your spouse or civil partner has transferred 10 per cent of his or her £12,500 (£1,250) personal allowance to you, known as the Marriage Allowance, reducing your tax bill by £250.
N: The other way round — you have transferred 10 per cent of your allowance to your partner.
NT: You pay no tax on any of your income.
0T: All your income is taxed. You could get this if you have change jobs and have not had a P45 showing how much tax you have paid so far this year.
S: Your income or pension is taxed at the Scottish rate.
T: Your tax code requires other calculations to work out your current personal allowance.
W1 or M1: Emergency tax code. HMRC needs more information.
More Details here https://www.gov.uk/tax-codes