Do you have anything to declare?
No nothing. Just the normal allowance.
I don't understand question and its answer.
Please tell me meaning and where this sentence used.
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This sounds like a conversation between a customs officer and a traveller arriving in a country. People arriving in the UK (for example) may bring certain items that are taxed but which are cheaper outside the country, up to a limit (the allowance). For example 42 litres of beer, 18 litres of wine, 9 litres of strong liquor, 200 cigarettes. Also other goods worth up to £390. If you have these items up to these amounts, you have the "normal allowance" and have "nothing to declare" to the customs officer. If you have more than the allowance of any of the items, you must declare them. You must also declare any forbidden or restricted items (weapons, drugs, endangered animals, obscene materials, goods that you plan to sell, etc). You may have to give up or pay tax on anything you declare. Lying to a customs officer is a crime in many countries.
The conversation is equivalent to this:
Customs officer: Do you have any items in excess of the allowances, or any forbidden or restricted items?
Traveller: No, only permitted items within the allowances.
The context seems to be a Customs official questioning a traveler at a point of entry. Travelers are required to declare (that is, to officially acknowledge that they possess) certain items like plant specimens, large amounts of cash, and so on.
The traveler responds that they have nothing to declare--they aren't bringing into the country anything they're required by law to let Customs know about. Many countries allow you to bring in, for example, a single bottle of wine for personal consumption without having to officially declare it. A single bottle is within the permitted amount--the "normal allowance." So the traveler is acknowledging that they may have a small quantity of something (it's not clear what) but that it is within the permitted amount.