In Brazil, Law No. 5,172, of October 25, 1966 lays down the general tax rules, which are applicable on a national level. This law is known as "Código Tributário Nacional" (meaning National Tax Code) or by the acronym "CTN".
There are multiple definitions - click on the translation that matches your context.
a number used by an employer or pension provider to calculate the amount of tax to deduct from your pay or pension
If you have the wrong tax code you could end up paying too much or too little tax.
the set of laws concerning tax
The dispute concerns the meaning of the word "directly" within Tennessee's tax code.