Some countries, such as Canada and the United States, may require the visitor to include a letter of invitation with their travel visa application. A letter of invitation is a formal letter from the person you intend to visit stating that they are inviting you to visit them in that country. Invitation letters help travel authorities vet potential visitors by making sure that a temporary visit is indeed the true nature of the visa request. Check with your intended destination’s government website for details on what needs to be included in a such a letter.

The Democratic People's Republic of Korea (North Korea) requires that its citizens obtain an exit visa stating the traveller's destination country and time to be spent abroad before leaving the country. Additionally, North Korean authorities also require North Korean citizens obtain a re-entry visa from a North Korean embassy or North Korean mission abroad before being allowed back into North Korea.
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