Your PAYE coding notice shows your tax code. The company uses the code to work out how much income tax to take off your salary through PAYE. Your tax office usually sends you a PAYE coding notice in January or February for the tax year beginning on the following 6 April. The tax code shown in the notice is for that tax year only. If your tax office is made aware of a change to your circumstances during the tax year, you will receive a revised PAYE coding notice.
Not everyone gets a PAYE coding notice each year. It depends on what allowances and reliefs you are claiming and whether these tend to change from year to year. If the only changes are the increase in the personal allowance or changes in tax rates, the company will include these automatically in your salary and you won't get a PAYE coding notice.
If the tax office don't know enough about your income or tax details they will give the company an emergency tax code. An emergency code assumes that you are only entitled to the basic personal allowance but it won't take into account any other allowances or reliefs you may be entitled to.