Immigration is the process by which people are authorized to cross international borders. This includes checks such as travel documents (passports, travel passes) and the ability to entry a country or territory (visas, and visa exceptions).
This tag should be used when your question involves general issues with customs and immigration. Customs and immigration covers border control issues and types of goods allowed to be brought into a country and taxation rules related to them, if any.
In the US 'Customs and Border Control' is collectively referred to by a single umbrella term 'Customs'.